TiO₂ nano-materials were synthesized by injecting acid solution (consisting of distilled water, ethyl alcohol and acetic acid) into tetrabutyl titanate. The mixture was kept under constant magnetic stirring for 3 h. In order to obtain nanoparticles, the sol was dried for 48 h. Sintering processes were subsequently carried out to obtain desired TiO₂ crystalline. The ion doped nanoparticles were synthesized with the same method mentioned above, except for the addition of the corresponding zinc and cerium doping solution into the acid solution.

The X-ray diffraction (XRD, Philips) equipped with a copper target ( λ_Kα1= 0.1541874 nm) was used to identify the formation phase of the obtained samples. The morphology and grain size of the nano-materials were analyzed with TECNAI G2 F20 transmission electron microscope (TEM). The binding energy was identified by X-ray photoelectron spectroscope (XPS) with MgKα radiation (ESCALAB250). Diffuse reflectance spectra (DRS) were measured by using a Shimazu UV-2500 spectrophotometer.

E. coli ( E. coli, ATCC 25922) as the model bacteria, antibacterial properties of Zn-Ce/TiO₂ nano-materials were tested by using inhibition zone method under visible light irradiation and in the dark. Observe the materials surrounding bacterial growth, measure transparent antibacterial circle diameter, and then determine antibacterial properties.

XRD was carried out to investigate the changes patterns TiO₂ phase structure after calcination treatment and zinc and cerium co-doping. Fig. 1 shows the effect of calcination temperatures on the phase structure of Zn-Ce/TiO₂ nano-materials. It is found that the (101) plane (peak 25.3°) is consistently present at all the calcination temperatures but the FWHM decrease with the increase of temperature. It indicates the crystal becomes larger with increase of calcination temperature. When the nano-material was calcinated at 650℃, Fig. 1 shows some characteristic peaks of CeO₂ phase (major peaks: 28.4°, 33.0°) and CeO₂ phases intensity increase gradually with the increase of calcination temperature. It is supposed that residual cerium component separated from the anatase phase is crystallized as CeO₂ phase. This is because of much lower solubility of cerium in the anatase phase when the calcination temperature increases. As calcination temperature reaches 700℃, the trace of cubic Zn₂Ti₃O₈ phases (major peak:35.4°) form. According to previous report, ZnO reacted with TiO₂ to transform into cubic Zn₂Ti₃O₈^{[ 9]}. At the same time, it is reported that Zn²⁺ cannot substitute for Ti⁴⁺ of [TiO₄] tetrahedrons in amorphous or [TiO₆^-] octahedrons in anatase and rutile crystals. At this occasion, Zn²⁺ mainly located in the discontinuity sites of the amorphous network, enriched around the crystals, or formed ZnO, Zn₂Ti₃O₈ and ZnTiO₃ phases. When calcination temperature reaches 800℃, Zn₂TiO₄ phases (major peaks: 35.4°) appear together with the rutile phases (major peaks: 27.4°). Obviously, the phase transformation from anatase to rutile occurs at 800℃. It also reveals that the doping could retard the transformation from anatase to rutile at elevated temperature.

Fig. 1

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	Fig. 1 XRD patterns of Zn/Ce-TiO2 calcined at different temperatures

To investigate the effect of zinc and cerium co-doping on the crystal structure of TiO₂, we calculated the lattice parameters of TiO₂ nano-materials listed in Table 1. It is clearly seen from Table 1 that the lattice parameters of all nano-materials remain almost unchanged along the a-axis. The c-axis parameters of co-doping TiO₂-550 decrease compare with pure TiO₂-550. This indicates that the crystal lattices of materials are locally destroyed by Zn and Ce doping. The ionic radii of Ce³⁺, Ce⁴⁺, Zn²⁺ and Ti⁴⁺ are 0.101, 0.087, 0.060 and 0.068 nm, respectively. Unlike the action of Zn²⁺, Ce³⁺ and Ce⁴⁺ with larger radius remaining in anatase crystals is nearly impossible, therefore, they should be located around anatase crystals^{[ 9]}.

Table 1

Table 1

Table 1 Lattice parameters of TiO2 nano-materials Materials a( b) /nm c / nm V / nm3 TiO2-550 0.37820 0.95076 0.13599 Zn-Ce/TiO2-550 0.37846 0.92187 0.13204 Zn-Ce/TiO2-600 0.37826 0.93116 0.13323 Zn-Ce/TiO2-650 0.37867 0.92702 0.13338 Zn-Ce/TiO2-700 0.37861 0.95734 0.13723 Zn-Ce/TiO2-750 0.37885 0.95971 0.13774 Zn-Ce/TiO2-800 0.37851 0.94746 0.13574

Table 1 Lattice parameters of TiO2 nano-materials

TEM images of Zn-Ce/TiO₂-600 are shown in Fig. 2. The morphology, particles sizes and crystallographic lanes have been verified using the TEM study. The TEM analysis has revealed that the grains are round-shaped. And after doping Zn-Ce, the particles sizes become smaller and better dispersion. A corresponding selected area electron diffraction pattern (SADP) shows the Debye rings exhibit a polycrystalline nature of the nano-materials. The Debye rings with d-values corresponding to 0.3512 nm (marked as (101)), 0.2293 nm (marked as (004)), 0.1841 nm (marked as (200)), 0.1626 nm (marked as (211)), 0.1442 nm (marked as (204)), and 0.1297 nm (marked as (215)) are assigned to the TiO₂ anatase phases, respectively.

Fig. 2

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	Fig. 2 TEM image of Zn-Ce/TiO2-600 (a) and SAED pattern of Zn-Ce/TiO2-600 (b)

X-ray photoelectron spectroscopy analysis has been applied as an important characteristic method to detect the chemical states of different elements in doped nano-materials. The XPS spectrum of Zn-Ce/TiO₂-600 in Fig. 3(a) indicates that the material contains Ti, O, C, Ce and Zn elements. The C element can be ascribed to the residual carbon from precursor.

Figure 3(b) shows the high-resolution XPS spectra of Ce3d. According to the literature [10], the Ce3d spectrum is complex and can be assigned 3d_3/2 spin-orbit states (labeled u) and 3d_5/2 states (labeled v). The u3/ v3 doublet is ascribed to the primary photoemission from Ce⁴⁺-O₂. The u/ v and u2/ v2 doublets are shake-down features resulting from the transfer of one or two electrons from a filled O2p orbital to an empty Ce4f orbital. The u1/ v1 doublet is attributed to photoemission from Ce³⁺. As can be seen from Fig. 3b, it could be assigned to 3d_3/2 spin-orbit states containing five main peaks at 899.37 ( u0), 901.91 ( u), 903.75 ( ul), 906.01 ( u2), 915.47 eV ( u3), and 3d_5/2 states containing five main peaks at 880.50 ( v0), 883.14 ( v), 884.70 ( vl), 888.49 ( v2), 896.62 eV ( v3). The Ce3d spectrum of Zn-Ce/TiO₂-600 nano-material shows a mixture of Ce in 3+/4+ oxidation states giving rise to these five pairs of doublets, indicating the coexistence of Ce³⁺and Ce⁴⁺ in Zn-Ce/TiO₂-600 nano-material. Nevertheless, diffraction peaks for Ce₂O₃ are not found in Fig.1, thereby it indicates that Ce₂O₃ might be amorphous^{[ 11]}.

Fig. 3

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	Fig. 3 XPS spectra for Zn/Ce-TiO2 materials(a) Zn/Ce-TiO2-600; (b) Ce3d regions

The UV-Vis diffuse reflectance spectrum of Zn-Ce/TiO₂ calcinated at different temperatures is illustrated in Fig. 4. It can be clearly observed that the absorption edge positions of all zinc and cerium co-doped materials extend into the visible region. It is clear that the incorporation of co-doped results in a red-shift of the absorption of TiO₂ nano-materials. The red shift of the absorption edge implies that the band gap energy decreases, the materials could absorb more photons, and eventually photoactivity of the materials increase. Assuming the materials to be indirect semiconductors, the plot of transformed Kubelka-Munk function vs the energy of light affords band gap energies of 2.63, 2.70 , 2.75, 2.8, 2.82 and 2.86 eV for 550℃, 600℃, 650℃, 700℃, 750℃ and 800℃, respectively (see the inset).

Fig. 4

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	Fig. 4 UV-Vis diffuse reflection spectra for Zn/Ce-TiO2 materials calcined at different temperatures(a) 550℃; (b) 600℃; (c) 650℃; (d) 700℃; (e) 750℃; (f) 800℃

Figure 5 shows the results of inhibition zone Zn/Ce- TiO₂ nano-materials. It can be seen from the Fig. 5(a), Zn/Ce-TiO₂ nano-materials exhibit strong antibacterial activity under the visible light. With the increase of the calcination temperature, antibacterial activities of Zn/Ce- TiO₂ are enhanced. Calcination temperature is 650℃, antibacterial activity of Zn/Ce-TiO₂ nano-material is best. Subsequently, antibacterial activities are weakened with increase of temperature. The average inhibition zones are 0, 5, 8.5, 7, 8 and 7 mm for Zn/Ce-TiO₂ obtained at 550℃, 600℃, 650℃, 700℃, 750℃ and 800℃, respectively.

Fig. 5

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	Fig. 5 Antibacterial results of Zn/Ce co-doped TiO2 at different calcination temperatures(a) Under visible light; (b) In the dark

In the Fig. 5(b), it also can be seen obscure inhibition zone. It shows Zn/Ce-TiO₂ nano-materials have slight antibacterial activity in the dark. But their antibacterial activities are worse in the dark than that under the visible light.

According to the results of antibacterial effect for Zn-Ce/TiO₂, we suppose that the antibacterial mechanism of Zn-Ce/TiO₂ is as follows:(1) It could be hypothesized that titanium zinc cerium, a semiconductor, would act in a similar manner, and would catalyze the formation of free radicals. Radical oxygen species can inhibit bacterial growth^{[ 12]}. XRD data proves Zn₂Ti₃O₈ phase exists in the materials. Wang, et al^{[ 13]} proposed that the number of Lewis acid sites enhanced in the material surface and OH^- increased, due to the cubic Zn₂Ti₃O₈ phase. OH^- can capture electron holes pair and transform into free groups (·OH), and cause rupture of the cell membrane.

(2)XPS data shows both Ce³⁺ and Ce⁴⁺ ions reside in the Zn/Ce-TiO₂ nano-materials. A mixture of Ce³⁺ and Ce⁴⁺ oxidation states giving rise to a myriad of peaks indicates that the surface of the material is not completely oxidized. It is supposed that the coexistence of Ce³⁺ and Ce⁴⁺ can improve the photocatalytic efficiency through the following ways^{[ 13]}:Ce⁴⁺+ e_cb^- → Ce³⁺

Ce³⁺+ O_ads → Ce⁴⁺+ O_ads^-

Ce⁴⁺ can trap the excited electron in conduction band, inhibiting the electron/hole pair recombination; meanwhile, Ce³⁺ can offer electron to the oxygen adsorbed on the surface of catalyst, thus the interfacial electron transfer is accelerated.

(3) XRD and XPS data also demonstrate that cerium xide exits in the particles surface. Moon, et al^{[ 14]} proposed the binary oxide catalysts often exhibited higher catalytic activity than what could be directly predicted from the properties of their components. The interface between the two phases could act as a rapid separation site for the photogenerated electrons and holes due to the difference in the energy level of their conduction bands and valence bands. Furthermore, DRS analysis proves absorption edge positions of Zn-Ce/TiO₂ materials extend into the visible region. It is also helpful to the photocatalytic reaction under the visible light.