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Applied Surface Science 292 (2014) 608– 614

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h omepa g e: www.elsev ier .com/ locate /apsusc

reparation and characterization of Zn/Ce/SO42−-doped titania

ano-materials with antibacterial activity

uzheng Wang, Xiangxin Xue, He Yang ∗, Che Luannstitute of Metallurgical Resources and Environmental Engineering, Northeastern University, Shenyang 110819, PR China

r t i c l e i n f o

rticle history:eceived 16 June 2013eceived in revised form5 November 2013ccepted 4 December 2013

a b s t r a c t

SO42−-doped Zn/Ce/TiO2 nano-materials (Zn/Ce/SO4

2−/TiO2) were prepared by a sol–gel method. Thestructures of Zn/Ce/SO4

2−/TiO2 nano-materials were characterized by Transmission electron microscope(TEM), X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), X-ray photoelectron spectroscopy(XPS), X-ray photoelectron (PL) spectroscopy and Fourier transform infrared spectroscopy (FT-IR). Gram-negative Escherichia coli (ATCC25922) and Gram-positive Staphylococcus aureus (ATCC6538) as model

vailable online 13 December 2013

eywords:ntibacterial activityO4

2−

organisms, antibacterial activities of nano-materials were tested using inhibition zone method and shak-ing flask method under visible light irradiation and in the dark. The results show that the materialscrystal structure and elemental composition are changed after SO4

2− doped. Zn/Ce/SO42−/TiO2 exhibit

predominant antibacterial activity in the dark and visible light irradiation. The action mechanism ofssed

n and Ce co-doped

iO2

Zn/Ce/SO42−/TiO2 is discu

. Introduction

In recent years, various antibacterial materials containing inor-anic mental atoms or ions such as silver have been developed1–4]. Compared to organic antibacterial materials, inorganicntibacterial materials have many excellent properties. TiO2 isonsidered to be an excellent photocatalyst, because it has manydvantages of other materials, such as high antibacterial activities,nexpensive and non-toxic [5–11]. However, the disadvantage ofhis material is that the wide band gap of TiO2 (3.2 eV) in lightbsorption can only absorb in the ultraviolet region, so that itsntibacterial activity is not suitable for commercial applications.o many efforts of doping TiO2 have been paid by the adding someetallic and non-metallic ions [12–14]. Rodrigo et al. [15] investi-

ated the ZnO could obviously enhance the antibacterial activity ofiO2 nano-materials. Akhavan et al. [16] introduced the grapheneoxide)/TiO2 thin films against the Escherichia coli bacteria with usef the so-called antibacterial drop-test. It pointed out the antibac-erial activity of the graphene (oxide)/TiO2 thin film was enhancednder solar light irradiation. Anchun et al. [17] prepared silver-ydroxyapatite/titania composite nanoparticles by sol–gel methodnd materials exhibited obvious antibacterial effect on E. coli andtaphylococcus aureus.

Solid acids is green catalyst and it has been widely used in a num-er of industrially important reactions. Recently, TiO2-based solidcids photocatalysts have been effectively introduced in the field of

∗ Corresponding author.E-mail address: yangh@smm.neu.edu.cn (H. Yang).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.12.017

.© 2013 Elsevier B.V. All rights reserved.

photocatalysis. Zhimou et al. [18] reported sulfated TiO2, known asTiO2–SO4

2− solid superacid, was easier to improve the hydrophilicproperties of TiO2. The surface acidity was considered to exist in theform of stronger surface hydroxyl groups and hydroxyl groups canaccept holes by illumination generated. Li et al. [19] also reportedSO4

2−-doped TiO2 photocatalyst prepared by treating the xerogelunder supercritical conditions. They pointed out that SO4

2−/TiO2exhibited higher activity and longer lifetime than the correspond-ing undoped TiO2. However, to our knowledge, the antibacterialactivity of SO4

2−/TiO2 has been seldom studied. In this contribution,it is assumed that the overall photocatalytic antibacterial activityof the material will be improved if its strong acidity and photocata-lysis are ideally combined. So, a synergistic effect of photocatalyticactivity and surface acidity can change the antibacterial activity ofthe material under the visible light irradiation and in dark.

According to the previous experimental results [20], Zn andCe co-doped TiO2 has significant antibacterial activity after highcalcinations temperature. So we are interested in the super acidifi-cation modification of material, stressing its effect on the structureand the photocatalytic antibacterial properties. In this paper,Zn/Ce/SO4

2−/TiO2 modified material are prepared by a sol–gelmethod and their antibacterial effect is examined against E. colior S. aureus under the visible light irradiation and in the dark.

2. Experimental

2.1. Preparation of Zn/Ce/SO42−-doped TiO2 nano-materials

A detailed description of the TiO2 sol preparation is given else-where [12]. The ions doped nanoparticles were synthesized with

face Sc

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2

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2

asfi

Y. Wang et al. / Applied Sur

he same method mentioned above, except for adding the corre-ponding Zn, Ce and SO4

2− solution into the acid solution. Thebtained xerogel was calcinated at 500 and 600 ◦C for 2 h to removeesidual organic compounds. The resulting materials were labeleds Zn/Ce/SO4

2−/TiO2-x, where x denoted the calcination tempera-ure (◦C). The molar ratio of Zn, Ce, SO4

2− and Ti is 3:2:5:50.

.2. Characterization

Transmission electron microscope (TEM, TECNAI G2 F20,SA) studies were used to characterized the morphology of

he Zn/Ce/SO42−/TiO2 materials. The X-ray diffraction (XRD,

hilips) patterns were performed to identify the formation phasef the obtained materials. The surface area of materials waseasured using nitrogen (N2) adsorption–desorption isothermeasurements at 77 K on a nitrogen adsorption apparatus (BET,uantachrome NOVA 1200e, USA). Photoluminescence (PL) spectraere measured at room temperature on a Fluorescence Spec-

rophotometer (F-7000, Hitachi, Japan). The excitation wavelengthas 325 nm, the scanning speed was 1200 nm/min, and the PMT

oltage was 700 V. The width of excitation slit and emission slitere both 5.0 nm. X-ray photoelectron spectroscopy (XPS, ESCALAB

50, USA) measurements were analyzed with a X-ray source ofl K� (hv = 1486.6 eV), operating at 15 kV and 150 W. The Zn(2p),e(3d), O(1s), Ti(2p) and S(2p) peaks were deconvoluted usingaussian components, after a Shirley background subtraction. Theourier transfer infrared (FT-IR) spectroscopy was collected on apectrophotometer (FT-IR, Quantachrome Nicolet 380, USA) usinghe standard KBr method. Atomic absorption spectrometric wasecorded on a Perkin Elmer M2100 spectrophotometer (AAS).

.3. Zn, Ce and SO42− ions release

0.1 g of dried powder of Zn/Ce/SO42−/TiO2 nano-materials was

dded into 25 mL deionized water and then the suspension washaken for 2 h at a speed of 60 rpm using an oscillator in the arti-cial climate box at 37 ◦C. The concentration of Zn and Ce ions in

Fig. 1. TEM images of Zn/Ce/SO42−-doped TiO

ience 292 (2014) 608– 614 609

the supernatants was obtained by atomic absorption spectroscopy(AAS) analysis. At the same time, the concentration of SO4

2− ionswas determined by gravimetric method.

2.4. Measurement of antibacterial activity

The experiments were carried out under two irradiation condi-tions: visible light and dark. The visible light source was 15 commonfluorescent tubes, mounted on the inside of an artificial climatebox (with total power of 15 × 18 W). The procedures of inhibitionring method were as follows. Pouring agar into Petri dishes to form4 mm thick layers and the Petri dishes were left for 10 min to dry inthe air. Dense inoculum of the tested microorganisms was addedto Petri dishes, and then the compacted powder (0.70 g of samples,14 mm in diameter) was arranged on the agar surface and incu-bated at 37 ◦C for 24 h in the artificial climate box under the visiblelight. A vernier caliper was adopted to measure the diameter of thewidth of inhibition zone (mm).

The procedures of shaking flask method were as follows. 0.1 g ofeach material was dispensed into 25 mL of a sterile 0.9 wt.% salinewater containing about 105–106 cfu/mL of E. coli or S. aureus, andthen shaken at 30 ± 1 ◦C. 0.5 mL of the suspension was taken outfrom the test tube after contact 120 min, and diluted to a certainvolume by 10-fold dilution. The diluted solution smeared on nutri-ent agar plates in triplicate and incubated at 37 ± 1 ◦C for 24 h. Thenumber of bacterial colonies on each plate was counted [21].

3. Results and discussion

3.1. Morphologies and phase

Fig. 1 shows the TEM images of the Zn/Ce/SO42−/TiO2 materials

calcinated at 500 ◦C and 600 ◦C. It is observed that calcination

temperatures influence the diameter and size distribution of TiO2materials. The average particle size of nanometer particles is about20 nm calcinated at 500 ◦C. When calcination temperature reaches600 ◦C, it is observed some agglomeration and crystallization

2 calcinated at (a) 500 ◦C and (b) 600 ◦C.

610 Y. Wang et al. / Applied Surface Science 292 (2014) 608– 614

F�

ppn(asp

tbccpioc1maatte

3

taaTZrTbfidecisbe

cies of SO4 coordinated to metals such as Zn or Ce (see theinset 4). With the temperature further reach 600 ◦C, the character-istic peaks of around 1280 cm−1 disappear, probably due to highcalcination temperature lead to the loss of SO4

2−.

ig. 2. X-ray diffraction patterns of the Zn/Ce/SO42−-doped TiO2. � – anatase TiO2,

– CeO2, � – Zn(OH)2, � – CeOSO4, � – ZnSO4, © – ZnS, � – Ce(SO4)2.

henomenon. The BET surface areas of the Zn/Ce/SO42−/TiO2 are

resented in Table 1. At these temperatures, Zn/Ce/SO42−/TiO2

ano-materials possess larger specific surface area than Zn/Ce/TiO2the BET surface areas of Zn/Ce/TiO2 calcinated at 500 and 600 ◦Cre 114.855 and 48.678 m2/g, respectively). So the increasedpecific surface area can improve photocatalytic antibacterialerformance of Zn/Ce/SO4

2−/TiO2.XRD analysis was employed to determine the crystal structure of

he materials. Fig. 2 shows XRD patterns of Zn/Ce/SO42−/TiO2 after

eing calcinated at elevated temperatures. The sharp peaks at 25.3◦

orrespond to anatase phase (1 0 1). Therefore, Zn/Ce/SO42−/TiO2

alcinated at 500 ◦C and 600 ◦C mainly exhibit anatase crystallinehase. But the crystallization degree increases with the increase

n calcination temperature from 500 ◦C to 600 ◦C. The broadeningf these peaks, show that the materials are nanosized, and cal-ulations based on the Scherrer equation show all particles to be5–20 nm in size. From Fig. 2, one can see that the SO4

2−-dopedaterials generate some new phases under calcinated temper-

tures. Material Zn/Ce/SO42−/TiO2-500 exhibits Ce(SO4)2, ZnSO4

nd a small amount of Zn(OH)2. When the calcination tempera-ure reaches 600 ◦C, phases structure also become complicated dueo heat decomposition of SO4

2−. Material Zn/Ce/SO42−/TiO2-600

xhibits CeOSO4, Zn(OH)2, CeO2, and ZnS.

.2. PL analysis

PL emission analysis has been often used to investigate theransfer behavior of the photogenerated electron–hole pairs, andlso, to understand the separation and recombination of the pairss the primary processes in the field of photocatalysis [22,23]he observed photoluminescence (PL) spectra of bare TiO2 andn/Ce/SO4

2−/TiO2 are shown in Fig. 3. The PL spectrum of mate-ial possesses two major peaks are around 380 and 470 nm [24].he peak observed at 380 nm is attributed to the bandgap recom-ination of TiO2, while other luminescence peaks in the rangerom 450 nm to 480 nm are related to transition from local-zed surface states to the valence band of TiO2. Generally, theecrease in the PL intensity can be accounted for the higherlectron–hole separation that gives long-lived photogeneratedharge carriers. In our study, the PL intensity of Zn/Ce/SO4

2−/TiO2

s low when compared to bare TiO2 which imply the suppres-ion of photogenerated electron recombination from conductionand to valence band in TiO2. The weakened PL intensity andnhance photocurrent Zn/Ce/SO4

2−/TiO2 nano-materials confirm

Fig. 3. PL spectra of the Zn/Ce/SO42−-doped TiO2 nano-materials.

the high efficiency of doping ions on suppression of photogeneratedelectron–hole recombination in TiO2 [25].

3.3. FT-IR analysis

The FT-IR spectra of Zn/Ce/SO42−/TiO2 materials are shown in

Fig. 4. The peaks around 500 cm−1 belong to Ti O stretching vibra-tion. The stretching vibration of C H bond has been identified at2900–3000 cm−1 [26]. A peak centered at 1100 cm−1 correspondingto C O C stretching [27]. Another peak near 1400 cm−1 could beattributed to NO3

− groups from the incomplete decomposition ofZn and Ce precursor. The peaks around 3400 cm−1 and 1650 cm−1

belong to OH bonds. Compared with bare TiO2, after doped TiO2,the hydroxyl peaks become stronger. It is suggested that doping canimprove the surface state of the material and generate more surfacehydroxyl groups. In the photocatalytic reaction, the more surfacehydroxyl groups of the materials, the more conducive to producemore hydroxyl radicals and improve the photocatalytic activity. Inaddition, the literature [28] reported the peaks around 1052, 1072,1124, 1158 cm−1 could be attributed to the characteristic frequen-

2− 2+ 4+

Fig. 4. FTIR spectra of Zn/Ce/SO42−/TiO2 nano-materials.

Y. Wang et al. / Applied Surface Science 292 (2014) 608– 614 611

) O 1s

3

oSir

CsdoptSsat

Fig. 5. XPS spectra for Zn/Ce/SO42−/TiO2 materials (a) Ce 3d spectrum, (b

.4. XPS analysis

The XPS spectra were used to further clarify the identificationf the material. The high-resolution XPS spectra of the Ce 3d, O 1s,

2p, Zn 2p, and Ti 2p region on the surface of material are shownn Fig. 5(a)–(e), respectively. The C element can be ascribed to theesidual carbon from precursor.

Fig. 5(a) shows the high-resolution XPS spectra of Ce 3d. Thee 3d spectrum can be assigned 3d5/2 states (labeled v) and 3d3/2pin–orbit states (labeled u). The u/v and u2/v2 doublets are shake-own features due to the transfer electrons from a filled O 2prbital to an empty Ce 4f orbital. The u1/v1 doublet ascribe tohotoemission from Ce3+ [29]. However, it is not detected thathe primary photoemission from Ce4+–O2 (916 eV), indicating that

O4

2− doping inhibits the formation of CeO2. The O 1s XPS (Fig. 5(b))pectra are asymmetric and wide, demonstrating that there aret least two kinds of chemical states. After curve fitting, besideshe peaks corresponding to metal–O (OM) and the chemisorbed

spectrum, (c) S 2p spectrum, (d) Ti 2p spectrum and (e) Zn 2p spectrum.

water (OH) [30], the peak at about 532.5 eV is related to oxygen(OS) in the [SO4

2−] crystal lattice. Furthermore, the XPS spectrumfor Zn/Ce/SO4

2−/TiO2 exhibits one peak in the S 2p region (Fig. 5(c)).It is indicated that the S 2p spectra are composed of 2p3/2 and 2p1/2peaks due to the spin–orbit splitting. Generally, the S 2p3/2 peakat lower binding energy is nearly twice the intensity of the higherbinding energy S 2p1/2 peak. These peaks are about 1.1 eV apartand cannot be resolved from one another [31]. From the previousreport [32], the S 2p3/2 peak in (NH4)2SO4, or ZnSO4 was 169.4 eV.It can be deduced that the S element is existence of the compoundZnSO4 or Ce(SO4)2. The Ti 2p spectrum in Fig. 5(d) exhibits twopeaks at 458.6 eV and 464.5 eV indicating that Ti is 4+ valence inthe doped TiO2 [33]. As shown in Fig. 5(e), the Zn 2p region con-sists of a spin–orbit doublet with Zn 2p3/2 having a binding energy

Eb about 1022 eV and Zn 2p1/2 with a Eb of 1045 eV, correspondingto the 2+ oxidation state in the doped system.

The atomic ratios of Ti 2p, Zn 2p, Ce 3d, O 2p and S 2p on thesurface are shown in Table 2. With the increase of the calcination

612 Y. Wang et al. / Applied Surface Science 292 (2014) 608– 614

Table 1Effect of SO4

2−-doped on phase structure.

Sample Phase composition Crystallite diameter (nm) BET (g/m2)

Zn/Ce/SO42−/TiO2-500 Anatase TiO2; Ce(SO4)2; Zn(OH)2; ZnSO4 15 129.54

Zn/Ce/SO42−/TiO2-600 Anatase TiO2; CeOSO4; Zn(OH)2; ZnS; CeO2 20 49.654

Table 2XPS data of different chemical states of six elements on the Zn/Ce/SO4

2−/TiO2 nano-materials.

Sample Surface atomic concentrations (%)

Ce S O Ti Zn C

Ce3+ Ce4+ Ce3+/Ce4+ OM OH OS

FS

Zn/Ce/SO42−/TiO2-500 0.987 1.60 0.617 8.04

Zn/Ce/SO42−/TiO2-600 0.550 1.05 0.524 5.5

ig. 6. Antibacterial experiments of Zn/Ce/SO42−/TiO2 and bare TiO2 nano-materials unde

taphylococcus aureus.

19.49 7.86 18.35 8.74 4.48 30.4520.07 24.09 10.73 5.47 32.54

r visible light irradiation by the inhibition zone method: (a) Escherichia coli and (b)

Y. Wang et al. / Applied Surface Science 292 (2014) 608– 614 613

Table 3The ions content from released solution of Zn/Ce/SO4

2−/TiO2 materials.

Sample no. pH Initial amount (mmol) AAS-determinedcontent (mmol)

Gravimetric method-determinedcontent (mmol)

Zn Ce SO42− Zn Ce SO4

2−

Zn/Ce/SO42−/TiO2-500 5.8 3 3 5 0.053 0.031 0.034

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Zn/Ce/SO42−/TiO2-600 5.7 3 3 5

emperature, Ce atomic concentration increased, but the atomicatio of Ce3+ and Ce4+ does not change. When the Ce4+ concentra-ion increased, the Ce4+ ion can capture the electrons to generate

large number of Ce3+; meanwhile, Ce3+ can also be trap holes,hereby indirectly the recombination of electrons and holes [20].

To further understand the relationship between the various ionsontent and their antibacterial activity, the amount of ions releasedrom the materials is measured by AAS and gravimetric method.hrough a series of calculations, the results are listed in Table 3.he amount of SO4

2− ions released from the materials cause theH value of the solution decreased.

.5. Antibacterial investigation

The prepared Zn/Ce/SO42−/TiO2 was tested for its antibacterial

ctivity under irradiation from a visible light source and dark condi-ion. The agar plates were inoculated with both S. aureus and E. coli.he antibacterial results are given in Figs. 6 and 7. Formation of inhi-ition zone confirms that all the Zn/Ce/SO4

2−/TiO2 nano-materialsave antibacterial activity in the visible light and in dark, while bareiO2 nano-materials do not have. According to results of shakingask method, first we have find that the surviving bacteria signifi-antly reduced in both in the visible light and dark conditions. Andhe antibacterial action of the material is more significant on E. colihan that on S. aureus. The different in activity against two types ofacteria can be explained by the chemical composition of the cellurfaces and different structures. The cell wall of E. coli is differ-nt from the S. aureus by having an outer membrane that covershe peptidoglycan layer. It also helps the bacteria to survive whenonsidering the presence of external substance that can harm theacteria [34].

2−

The reasons for antibacterial activity of Zn/Ce/SO4 /TiO2 areelieved to be due to the surface hydroxyl groups, the small par-icle size of the materials, changes of the crystal structure and the

aterial acidic enhanced after SO42−-doped. The first two factors

ig. 7. Antibacterial experiments of Zn/Ce/SO42−/TiO2 under visible light irradiation

nd in dark by shaking flask method: (a) Escherichia coli and (b) Staphylococcus aureusblank bacterial count was 106).

0.045 0.032 0.046

have been introduced in many literatures [35,36]: (1) after irradia-tion with visible light, generating oxidative species such as •OH onthe surface of the materials can effectively trap the holes. The highdensity of •OH provides more surface sites for adsorbing organicmolecules (such as the cell membrane of the bacteria) and effi-ciently captures the holes. This greatly prohibits the electron–holepair recombination, thereby improving the photocatalytic quan-tum efficiency [19] and leading to cell death. PL analysis has provedthat Zn, Ce and SO4

2− ions doping suppress photogenerated elec-tron recombination, (2) the small size of the nanoparticle is shownby the broadening peaks of the XRD and TEM image. The smallerthe particle size, the larger the specific surface area. A larger surfacearea for contacting with the bacteria also increases the antibacterialactivity of the Zn/Ce/SO4

2−/TiO2 and (3) according to our group pre-vious result [20], the phases of the Zn/Ce/TiO2 are mainly ZnTiO3,CeO2 and TiO2. However, in this paper, according to the XRD result,Ce(SO4)2, ZnSO4 and ZnS phase occurred due to SO4

2− doped. Hein-laan et al. [37] pointed out that both ZnO and nano ZnO were muchless toxic than ZnSO4 although the amount of soluble Zn should becomparable. The acute toxicity of both Zn oxides for Vibrio fischeriwas almost identical being 6-times less toxic than ZnSO4. On theother hand, ZnS has a great potential as an antibacterial materialdue to its relatively deeper conduction and valence bands. The com-paratively negative positions of the conduction and valence bandsof ZnS are located at −2.3 and +1.4 eV, respectively. Literature [31]reported that the donor levels, provided by the S and Zn vacancies,act as stepping stones for electrons traveling form the valence bandto the conduction band. The donor levels energy gaps between thevalance band and conduction bands are 2.5 eV, which fall insidethe region for visible light absorption. XPS data shows both Ce3+

and Ce4+ ions reside in the Zn/Ce/SO42−/TiO2 nano-materials. A

mixture of Ce3+ and Ce4+ oxidation states indicates that the sur-face of the material is not fully oxidized. It is supposed that thecoexistence of Ce3+ and Ce4+ can improve the photocatalytic effi-ciency. (4) That S. aureus and E. coli has an efflux mechanism forSO4

2− suggests that, on the surface of the material, SO42− gener-

ated by the dissociation of Ce(SO4)2 or ZnSO4 is toxic to the bacteria.And from AAS analysis result, there is a lot of SO4

2− in the releasesolution of Zn/Ce/SO4

2−/TiO2. On the one hand, the sulfated TiO2is known as a super-strong solid acid catalyst. Even in dark, thestrong acidic sites are conceived to carry out catalytic reaction ofa large number of organic compounds [38]. The strong acidic sitesalso may generate more acidic sites on the TiO2 surface, inhibi-ting the build-up of photo-induced electrons and holes. Thus, thequantum efficiency of the TiO2 photocatalyst is improved, result-ing in the higher photocatalytic activity. In addition, the growth ofbacteria requires appropriate environmental factors (temperature,pH, oxidation reduction potential, DO, solar radiation, the wateractivity and osmotic pressure and surface tension, etc.) in additionto the nutrients. The suitable pH value for each strain is different.Optimum pH of most bacteria is 6.5–7.5. However, in this paper,the pH value of the release solution is determined in the vicinity

of 6.0, beyond the optimum pH. This affects the enzyme activityand cell membrane temperature, thus affecting the microorgan-isms on the absorption of nutrients, is not conducive to the growthof bacteria.

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14 Y. Wang et al. / Applied Sur

. Conclusion

1) Zn/Ce/SO42−/TiO2 nano-materials were synthesized success-

fully by sol–gel method under different calcination conditions.The analysis of BET and TEM show that the particles of allantibacterial materials are nanoscale. XRD spectra confirmmaterials with the SO4

2−-dopant under calcinated temper-atures generate some new phases, such as Ce(SO4)2, ZnSO4and ZnS, Which have contributed to the antibacterial activ-ity of the material. PL measurements reveal that dopingions can efficiently restrain charges recombination so thatZn/Ce/SO4

2−/TiO2 nano-materials exhibit superior photocat-alytic activity. XPS and FT-IR spectra confirm SO4

2−-dopant canimprove the surface state of the material and generate moresurface hydroxyl groups.

2) The results obtained by the inhibition ring method and shakingflask method demonstrate that Zn/Ce/SO4

2−/TiO2 antibacte-rial materials can destroy or inhibit the growth of E. coli andS. aureus. Marked antibacterial activity of Zn/Ce/SO4

2−/TiO2 isattributed to surface hydroxyl groups, the small particle size ofthe materials, changes of the crystal structure and the materialacidic enhanced by SO4

2− doped.

cknowledgment

We gratefully acknowledge the support from National Naturalcience Foundation of China (No. 51090384).

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