Probing Photocatalytic Characteristics of Sb-Doped TiO 2 under Visible Light Irradiation

Sb-doped TiO 2 nanoparticle with varied dopant concentrations was synthesized using titanium tetrachloride (TiCl 4 ) and antimony chloride (SbCl 3 ) as the precursors. The properties of Sb-doped TiO 2 nanoparticles were characterized by X-ray diffraction (XRD), scanning electronmicroscope (SEM), fluorescence spectrophotometer, andUv-vis spectrophotometer.The absorption edge of TiO 2 nanoparticles could be extended to visible region after doping with antimony, in contrast to the UV absorption of pure TiO 2 . The results showed that the photocatalytic activity of Sb-doped TiO 2 nanoparticles was much more active than pure TiO 2 . The 0.1% Sb-doped TiO 2 nanoparticles demonstrated the best photocatalytic activity which was better than that of the Degussa P25 under visible light irradiation using terephthalic acid as fluorescent probe. The effects of Sb dopant on the photocatalytic activity and the involved mechanism were extensively investigated in this work as well.


Introduction
In the past decade, TiO 2 has become a hot spot of research due to its outstanding photoelectric properties, high chemical stability, low cost, and nontoxicity and can be widely applied on photons device [1], photocatalysis [2], sensor [3], and so forth.Although TiO 2 has many excellent properties, the application is limited due to a relatively wide band gap (3.2 eV for the anatase) and the quick recombination of electron-hole pairs in TiO 2 taking place on a time scale of 10 −9 -10 −12 s [4].Therefore, several methods have been attempted to enhance its photocatalytic activity, such as modifying TiO 2 with metals [5][6][7], nonmetals [8][9][10], or other semiconductors [11,12].In addition, the wide band gap limits the absorption of catalysts to UV light only.It has been reported that the spectral response of TiO 2 photocatalysts can be extended to visible light region through the doping with other elements in order to make a donor or an acceptor level [13,14] in the forbidden band of TiO 2 .The doping level can either capture excited electrons of TiO 2 from valence band or make electrons jump to the conduction band of TiO 2 .As a result, the long wave photons can be absorbed and the scope of absorption spectra region is extended to lower energy [15].Previously, TiO 2 has been doped with a variety of elements, such as N [16], Fe [17], La [18], and so forth.The effect of Sb element on photocatalytic activity under visible light irradiation has also been studied by several research groups, showing high visible light absorption with UV-vis absorption spectrum measurements [19].Although catalytic properties of Sb-doped TiO 2 have been found more effective, the photocatalytic mechanisms under visible light are not definitively clarified or elaborated.
In order to further study the catalytic properties of doped TiO 2 under visible light and the contribution of different free radicals in the catalytic reaction.In this work, pure and 0.1%∼5% Sb-doped TiO 2 photocatalysts were prepared by coprecipitation method from TiCl 4 and SbCl 3 .Moreover, the influence of Sb dopant on the structure of TiO 2 , the photoabsorption properties, and catalytic activity of these samples were studied systematically.Sb doping not only expands the absorption spectra of catalysts from UV light to visible light but also improves the catalytic activity of photocatalysts.The obtained results indicate that the catalytic activity of Sb-doped TiO 2 is better than Degussa P25 which absorbs the UV light only [20].Furthermore, because hydroxyl radicals (OH • ) and superoxide anion radical (O 2−• ) 2 Journal of Nanomaterials can be generated on the surface of pure and Sb-doped TiO 2 under visible light irradiation (>420 nm), the terephthalic acid was employed as the fluorescent probe to evaluate the photocatalytic activity of catalyst [21].For the understanding of the role of two kinds of free radicals in the catalytic process and the catalytic mechanism involved, scavengers of DMSO and p-benzoquinone are used to eliminate hydroxyl radicals (OH • ) and superoxide anion radical (O 2−• ), respectively.The distilled water from Milli-Q system was used in the preparation of materials.

Experimental
Pure and 0.1%∼5% Sb-doped TiO 2 nanoparticles were synthesized by coprecipitation method according to the literature [19], in which the TiCl 4 dissolved in diluted HCl with deionized water and the obtained mixture solution was named Solution A. Dissolving SbCl 3 in HNO 3 at a doping level ranging from 0.1% to 5% nominal atomic against TiCl 4 , we obtained Solution B. Vigorously stirring Solution B until it is completely dissolved and then adding Solution B to Solution A under vigorous stirring at room temperature, titanium hydrous gels were precipitated upon neutralization with NH 4 OH at pH∼8.The resulting precipitates were repeatedly washed to remove undesirable anions such as Cl − .The coprecipitated wet gels were treated in a butanol at 100 ∘ C for 2 h, followed by drying at 120 ∘ C for 10 h.The dried precipitates were calcined at 500 ∘ C in air to maintain the anatase phase of TiO 2 and milled with agate mortar in ethanol for 30 min.

Characterization of Catalyst. X-ray diffraction (XRD,
DX-2700) patterns collected from 10 ∘ to 80 ∘ in 2 with 0.02 ∘ steps/s were used to identify average crystallite sizes and crystallinity of the nanoparticles using a powder X-ray diffractometer ( = 0.154056 nm) with Cu K1 radiation from the monochromatized X-ray beam at 40 kV and 40 mA.The optical absorption spectra were measured using a UV-vis spectrophotometer (Shimadzu, UV-2500) equipped with an integrating sphere, a referenced against the compressed BaSO 4 powders.Particle morphologies of samples were examined by scanning electron microscopy (SEM, JSM 6301F) at 15 kV.The Brunauer-Emmett-Teller (BET) surface area ( BET ) of the samples was determined by nitrogen adsorption/desorption isotherm measurements at 77 K (ASAP 2010).The fluorescence emission spectrum was recorded at room temperature with excitation at 320 nm on a fluorescence spectrophotometer (Hitachi, F-4500).

Photocatalytic Activity Test.
The fluorescence probe sensitive to the free radicals was applied to detect the photocatalytic activity of pure and 0.1%∼5% Sb-doped TiO 2 nanoparticles.Terephthalic acid as the fluorescent probe can readily react with hydroxyl radicals (OH • ) and superoxide anion radical (O 2−• ) to produce highly fluorescent products [22].As shown in Scheme 1, the main products of terephthalic acid are 2-hydroxyterephthalic acid (2-HTA) or hydroxybenzoic acid (4-HBA), and only 2-HTA is highly fluorescent and thus can be easily determined by fluorescence spectroscopy [22,23].
Photoreactivity experiments: 5 mg photocatalysts of P25 TiO 2 ; pure TiO 2 ; and 0.1%, 0.5%, 1%, and 5% Sb-doped TiO 2 were suspended in 20 mL aqueous solution containing 0.01 M NaOH (pH∼11.5)and 500 M terephthalic acid, respectively.Before exposure to visible light, the suspension was stirred in the dark for 30 min.3 mL of the solution was then taken out after irradiation for 20 min and centrifuged for fluorescence spectroscopy measurement.The emission intensity of the 2-HTA was monitored with the excitation at 320 nm.The photoreactivity in visible region was investigated with the irradiation of light source ( > 420 nm) obtained by the cutoff filter at 420 nm from a 500 W Xe lamp.
In this way, we were thus able to study separately the reactivity of two radicals h + /OH • and e − /O 2−• on the surface of Sb-doped TiO 2 by observing the fluorescent spectra of products.
e − e − e − Figure 2(A) shows the UV-vis absorption spectra of pure and 0.1%∼5% Sb-doped TiO 2 prepared by coprecipitation method.The pure TiO 2 had strong absorption only in the UV region corresponding to its band gap energy, while Sbdoped TiO 2 exhibited a new absorption band in the short wavelength region of visible light.The visible light absorption was enhanced by the dopant of pure TiO 2 and increased with Sb dopant.The color of the prepared catalysts changed from yellowish-white to yellow with the increasing dopant concentration of Sb.
Figure 2(B) shows X-ray diffraction patterns of TiO 2 with different Sb doping concentration; the anatase is the crystal phase of samples prepared by coprecipitation method.The average crystallite sizes of TiO 2 can be estimated from XRD spectra by using the Scherrer equation [26]: where  is the crystallite size (nm),  is the Scherrer constant ( = 0.89),  is the wavelength of the X-ray radiation source ( = 0.154056 nm in this case),  is the full width at half maximum intensity (radians), and  is the angle between the incident and diffracted beams.The average crystallite sizes of TiO 2 are presented in Table 1.It is found that the average crystal sizes of TiO 2 with different Sb doping concentration  [27].The ionic radii of Ti 4+ and Sb 5+ are 0.0600 and 0.0605 nm, respectively [28], and the ionic radius of Sb 5+ is similar to that of Ti 4+ .It is suggested that Sb 5+ ions are likely be substituted at Ti 4+ sites within TiO 2 .Sb 5+ ions partially replace Ti 4+ until the solubility limit is reached.Figures 2(C) and 2(D) show the SEM images of pure and 1% Sb-doped TiO 2 , respectively.They indicate that the surface morphologies of pure and 1% Sb-doped TiO 2 are almost undifferentiated.All samples are irregular in shape, and the size of particles is in the range of 25∼40 nm.The particle sizes of 1% Sb-doped TiO 2 are almost the same as that of pure TiO 2 due to the small amount of Sb [29].There is a slight difference between the results of SEM and Scherrer formula, probably arising from aggregated particles.In addition, the specific surface area of the samples was obtained using the BET surface area measuring apparatus at the boiling point of liquid nitrogen.The specific surface area ( BET ) of catalyst is among 40.606∼69.220m 2 /g (as shown in Table 1).
Here, on the basis of the fact that both of these radicals h + /OH • and e − /O 2−• can react oxidatively with nonfluorescent probe terephthalic acid to generate product 2-HTA (highly fluorescent) or 4-HBA (Scheme 1), the catalytic activities can be monitored by fluorescence spectroscopy.Figure 3(a) shows fluorescence spectrum of terephthalic acid solution with P25, pure, and 0.1%∼5% Sb-doped TiO 2 , respectively.The results indicate that the samples of 0.1%∼1% Sb-doped TiO 2 have better photocatalytic activity compared with pure and Degussa P25 TiO 2 in visible light region, and 0.1% Sb-doped TiO 2 demonstrated the best photocatalytic activity.It could be suggested that Sb dopant acts as electron traps retarding electron-hole recombination and enhancing interfacial charge carriers transfer to the surface of the particles; however, the recombination rate will increase when the dopant concentration of Sb is too high [30].Upon excitation with visible light, electrons are excited from oxygen atoms in the conduction band, probably to the Ti 3d orbitals, and are further trapped by adsorbed O 2 to produce O 2−• radials.The photogenerated holes are trapped by adsorbed H 2 O or OH − to produce OH • radicals bound to the surface [25].These trapped electrons (e − /O 2−• ) or holes (h + /OH • ) react with the nearby adsorbed molecules, as shown in Figure 1.
It is known that the photocatalytic process will generate two kinds of free radical, e − /O 2−• and h + /OH • , to drive the subsequent chemical reaction.Here, on the basis of that e − /O 2−• and h + /OH • can be quenched by scavengers of pbenzoquinone and DMSO, respectively; it is expected to get more insight into the catalytic mechanism of pure and Sbdoped TiO 2 by adding scavengers.As shown in Figure 3(b),

Conclusion
The pure and Sb-doped TiO 2 photocatalysts have been prepared by a coprecipitation method using TiCl 4 and SbCl 3 as precursor.The influence of Sb doping concentration on the catalytic property, using terephthalic acid as fluorescent probe, has been studied in this work.Sb ions can be isomorphously substituted into the TiO 2 lattice to generate a new doping energy level and change the electron transition way of TiO 2 from valence band to conduction band.In this case, it extends the scope of TiO 2 absorption spectrum to the visible light region and improves photocatalytic activity of catalysts.The obtained results show that the photocatalytic activity of 0.1%∼1% Sb-doped TiO 2 is higher than pure TiO 2 , and 0.1% Sb-doped TiO 2 presents the best photocatalytic activity, which is much better than Degussa P25.This superior catalytic activity mainly arises from the fact that the Sb-doped TiO 2 generates a new level in the forbidden band of TiO 2 and contributes to the capture of carriers as well as improving the separation of photogenerated electron hole.As for the catalytic mechanism of OH • and O 2−• in the fluorescence probe method, it can be attributed to the product fluorescence bursts from the oxidation of terephthalic acid to form 2-HTA by either one of these two radicals.

Scheme 1 :
Scheme 1: Reactions of hydroxyl and superoxide anion radicals with terephthalic acid to form the 2-HTA or 4-HBA.

Figure 1 :
Figure 1: The generation of photogenerated electron-hole pairs and the formation process of two kinds of radicals on the surface of Sbdoped TiO 2 catalyst.

Figure 3 :
Figure 3: (a) Plots of the fluorescence spectral for the 500 M terephthalic acid on P25 TiO 2 and pure and 0.1%∼5% Sb-doped TiO 2 catalysts after visible light irradiation for 20 min.(b) The fluorescence intensity at 426 nm after irradiation 10 min for terephthalic acid on the pure and 0.1%∼5% Sb-doped TiO 2 , without scavenger (red), with DMSO (blue) and p-benzoquinone (green), respectively.

Table 1 :
Crystalline properties of the prepared TiO 2 photocatalysts.