# Ta/TiO2-and Nb/TiO2-Mixed Oxides as Efficient Solar Photocatalysts: Preparation, Characterization, and Photocatalytic Activity

Authors: Hussein Znad; Ming H. Ang; Moses O. Tade
Journal: International Journal of Photoenergy
 (2012)

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## Abstract

Ta/TiO2- and Nb/TiO2-mixed oxides photocatalysts were prepared by simple impregnation method at different TiO2 : Nb or Ta mass ratios of 1 : 0.1, 1 : 0.5, and 1 : 1, followed by calcination at 500∘C. The prepared powders have been characterized by XRD, XPS, UV-Vis spectra, and SEM. The photocatalytic activity was evaluated under natural solar light for decolorization and mineralization of azo dye Orange II solution. The results showed that Nb/TiO2- and Ta/TiO2-mixed oxides have higher activity than the untreated TiO2 under natural solar light. The maximum activity was observed for Nb/TiO2 sample (at mass ratio of 1 : 0.1), which is characterized by the smallest crystalline size (17.79 nm). Comparing with the untreated TiO2, the solar decolorization and mineralization rates improved by about 140% and 237%, respectively, and the band gap reduced to 2.80 eV. The results suggest that the crystal lattices of TiO2 powder are locally distorted by incorporating Nb5+ species into TiO2, forming a new band energy structure, which is responsible for the absorption in the visible region. Unlike Ta/TiO2, the Nb/TiO2-mixed oxides can prevent the grain size growth of the treated TiO2, which is important to achieve high solar photoactivity.


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## Body

## 1. Introduction

Titanium dioxide (TiO2) is nontoxic, efficient photo-catalyst, chemically stable, and relatively inexpensive. Although TiO2 is the most popular photocatalytic material, it has not been applied widely in the field of environmental pollution control under solar light. The band gap (Eg) of TiO2 anatase is ~3.2 eV and lies in the UV range so that only 5–8% of sunlight photons have the required energy to activate the catalyst [1, 2]. This relatively large band gap has significantly limited its application, particularly under solar and/or visible light. An effective way to improve the TiO2 photocatalytic activity is to introduce foreign metal ions as dopants into its lattice. Depending on the dopant type and concentration, the band gap of TiO2 can be tailored to extend the photo responsiveness into the visible light region. Usually, the UV activity of undoped TiO2 is much greater than the visible light activity of the doped material. Therefore, for solar applications, the photocatalysts should be tested under simulated solar irradiation or under real sun conditions [3].TiO2 doped with Tantalum (Ta) and niobium (Nb) by sol-gel method has been widely used in gas sensing film application [4–6]. Generally those metal dopants (Ta and Nb) are used in sensor applications to inhibit the TiO2 phase transformation from anatase to rutile and to hinder grain growth during heating. Niobium oxide and, in particular, its most stable form, Nb2O5, is an interesting semiconductor with a band gap of about 3.9 eV (decreasing to about 3.5 eV in the amorphous state), with a high dielectric constant and high index of refraction, which has found important applications in electronics and optical applications, including thin films (antireflective coatings, solar control, etc.) [7].Atashbar et al. [8] prepared thin films of TiO2 doped with niobium oxide, for use in oxygen sensing applications. Thick film gas sensors made by Nb- or Ta-doped TiO2 have also been prepared and investigated for atmospheric pollutant monitoring [5]. Vanadium and tantalum-doped titanium oxide (TiTaV) as a novel material for gas sensing has been prepared by Carotta et al. [4]. Furubayashi et al. [9] pointed out that Nb-doped anatase TiO2 film has excellent electrical conductivity and transparency.Technological interest in Nb-doped TiO2 derives from the fact that Nb doping leads to enhanced photocatalytic activity in the destruction of organochloride pollutants such as dichlorobenzene [10]. Some investigators [11–13] reported that photocatalytic activity of Nb-doped TiO2 was much better than that of pure TiO2. Castro et al. [11] prepared Nb-doped TiO2 by a simple, lowcost, and low-temperature hydrothermal technique. The Nb-doped TiO2 was more active than undoped TiO2 for diquat degradation under UV light irradiation. Yang et al. [14] prepared micrometer-sized Nb-doped TiO2 porous spheres by ultrasonic spray pyrolysis method. Comparing with pure TiO2, the prepared Nb-doped TiO2 enhanced photocatalytic activity for the photodegradation of aqueous methylene blue solution under both visible and solar light irradiation. The enhanced photocatalytic activities of Ta codoped TiO2 thin films under visible light have also been reported by Obata et al. [15].However, to our knowledge, the photocatalytic activity of Nb/TiO2- and Ta/TiO2-mixed oxides for wastewater treatment under natural solar light, has not yet been reported. Furthermore, modifying the commercial TiO2 (Degussa P25), which is relatively cheap and commercially available, to improve its solar photoactivity are an interesting subject.Very limited attempts were found recently in the literature to improve the Degussa P25 photoactivity [16, 17]; in both cases, the photocatalyst activity was tested only under UV light. For photocatalytic hydrogen production, Yu et al. successfully improved the commercial P25 TiO2 by impregnation with copper nitrate followed by calcinations at 350°C [18], and by a simple precipitation method using Ni (OH)2 clusters [19]. Zhao et al. [20] fabricated ordered titanate nanoribbon (TNR)/SnO2 films by electrophoretic deposition (EPD) using hydrothermally prepared titanate (P25 Degussa) as a precursor. The photocatalytic activity was evaluated by decolorization of RhB aqueous solution under UV irradiation. Shang et al. [21] prepared N-doped TiO2 powder by annealing commercial TiO2 (P25) under an NH3 flow at 550°C. 4-chlorophenol decomposition was used to evaluate the photocatalytic activity under the visible light irradiation. Qi et al. [22] and Yu et al. [23] used the commercial P25 to prepare Pt/TiO2 catalysts and CdS-sensitized Pt/P25 for photocatalytic hydrogen production. Recently, Znad and Kawase [1] successfully doped the commercial TiO2 (Degussa P25) with a nonmetal dopant (sulfur) and improved its solar photo-catalytic activity for decolourizing polluted wastewater.In the present work, with the aim of developing more efficient and cost-effective solar photocatalyst and to investigate the metal ions (such as Nb and Ta) influence, the Ta/TiO2- and Nb/TiO2-mixed oxides photocatalysts have been prepared by simple impregnation method using the well-known TiO2 (Degussa P25) as a Ti precursor and, respectively, niobium oxide (Nb2O5) and tantalum oxide (Ta2O5) as a metal oxides. Nb and Ta ions were chosen since these elements have ionic radii similar to that of Ti, so that they could replace it in a substitutional solution with negligible distortion of the lattice. Concentrations higher than the solubility limits have been chosen to study the possible segregation of a mixed oxide.This work was mainly devoted to find out whether the metal ions (Nb and Ta) can improve the photocatalytic activity (or properties) of TiO2 under natural solar light, beside its well-known activity in film sensing application.


## 2. Experimental

### 2.1. Preparation of Ta/TiO2- and Nb/TiO2-Mixed Oxides Photocatalyst

The Ta/TiO2- and Nb/TiO2-mixed oxides photocatalysts were prepared by a simple impregnation method. The mass ratios for the different samples are shown in Table 1. For comparison, untreated Degussa P25 TiO2 (pure TiO2) was used in this work. Degussa P25 is a mixture of anatase and rutile (80 : 20) with a BET surface area of about 52.4 m2/g and average primary particle size of about 20 nm.Table 1
Composition of prepared samples.
Sample referenceTiO2 : Nb (mass ratio)Nb molar %Sample referenceTiO2 : Ta (mass ratio)Ta molar %Pure TiO21 : 00Pure TiO21 : 00TNb11 : 0.12.9TTa11 : 0.11.8TNb21 : 0.513TTa21 : 0.58.3TNb31 : 123TTa31 : 115.3In a typical preparation (described for sample TTa2 = 1 : 0.5 as representative of the series), Tantalum oxide Ta2O5 (0.5 g, 1.13 mmol) was suspended in 30 mL ethanol under stirring. 1 g (12.52 mmol) of Degussa P25 TiO2 powder was suspended in 30 mL ethanol and the suspension then sonicated for 1 h at room temperature. The two suspensions of TiO2 and Nb2O5 or Ta2O5 were mixed under vigorous stirring for 4 hours at room temperature and then dried under vacuum to obtain a powder which was further dried in oven for 8 h at 80°C. The obtained powder, was ground and then calcinated at 500°C for 3 h in the presence of air with a ramp rate of 2°C/min.


### 2.2. Characterization

The crystalline structure of the prepared Ta/TiO2- and Nb/TiO2-mixed oxides photocatalysts was characterized by X-ray powder diffraction (XRD) analysis (RINT Ultima III, Rigaku co., Japan) using Cu Ka radiation at a scan rate of 4 degree/min. The acceleration voltage and the applied current were 40 kV and 40 mA, respectively. The mean size of crystallite (D, nm) was calculated from full-width at half maxima (FWHM) of corresponding X-ray diffraction peaks using Scherrer’s formula D=0.89λ/βcosϕ, whereλ is the wavelength of the X-ray radiation (λ = 1.54056 nm Cu Ka), β is the full-width at half maximum (rad), and ϕ is the reflect angle. The content of anatase wA(%) was determined according to the following equation [24]: wA(%)={IA/(IA+1.265IR)}×100, where IR and IA are the intensities of the diffraction peaks of rutile (110) and anatase (101), respectively, obtained from XRD patterns. X-ray photoelectron spectra (XPS) of the powders were measured using Shimadzu ESCA 750 photoelectron spectrometer with an MgKα 1253.6 ev; the shift of binding energy due to relative surface charging was corrected using the C1s level at 285 eV as an internal standard. The diffuse reflectance UV-Vis spectra were measured with an Ocean Optics high-resolution HR4000 USB spectrometer (HR4000 Ocean Optics Inc.) with an extended wavelength range from 190 to 1100 nm. The microstructures of the samples were observed by a scanning electron microscope, SEM (JSM-5310LV, Jeol Co., Japan).


### 2.3. Photocatalytic Activity Measurement

Solar photocatalytic decolorization and mineralization of Orange II dye was carried out in 300 mL Pyrex glass beaker. All the reactions were carried out according to the following procedure: 0.4 g of photocatalyst (modified or unmodified TiO2) was added to 250 mL of aqueous dye (Orange II) solution (20 mg/L). The dyes solution was mixed with a magnetic stirrer during the course of the experiment. At the beginning, the solution mixture was stirred for 30 min in the dark to ensure establishment of the dye’s equilibrium adsorption/desorption. Then sun light was allowed to irradiate the reaction mixture, and at regular time intervals, samples were taken from the suspension and the change of Orange-II concentration was measured using UV-Vis spectrophotometer (TU-1900 UV spectrometer) at fixed wavelength of 486 nm. For this purpose, the photocatalyst was immediately removed from the sample by filtration, using a 0.4 μm syringe filter. Solar experiments were carried out from 10:00 AM to 5:00 PM during May in Saitama, Japan. During the solar experiments, the UV and the visible ranges (illuminance) of the solar light intensity were measured by the UV radiometer (UVR-2, TOPCON, with UD-36 (310–400 nm/average 365 nm) detector, Tokyo, Japan) and illuminance meter (T-10, Konica Minolta), respectively. The accumulated solar energy was calculated by [25]
(1)Qn=Qn-1+ΔtI(AV),Δt=tn-tn-1.Qn accumulated solar energy per unit of slurry volume (KJL-1), Δtis the time difference between radiation measurements (h), I is the solar light intensity per unit of irradiation surface area measured during time interval Δt (Wm−2), A is the irradiated surface area of the photoreactor (m2), and V is the photoreactor volume (m3).


## 2.1. Preparation of Ta/TiO2- and Nb/TiO2-Mixed Oxides Photocatalyst

The Ta/TiO2- and Nb/TiO2-mixed oxides photocatalysts were prepared by a simple impregnation method. The mass ratios for the different samples are shown in Table 1. For comparison, untreated Degussa P25 TiO2 (pure TiO2) was used in this work. Degussa P25 is a mixture of anatase and rutile (80 : 20) with a BET surface area of about 52.4 m2/g and average primary particle size of about 20 nm.Table 1
Composition of prepared samples.
Sample referenceTiO2 : Nb (mass ratio)Nb molar %Sample referenceTiO2 : Ta (mass ratio)Ta molar %Pure TiO21 : 00Pure TiO21 : 00TNb11 : 0.12.9TTa11 : 0.11.8TNb21 : 0.513TTa21 : 0.58.3TNb31 : 123TTa31 : 115.3In a typical preparation (described for sample TTa2 = 1 : 0.5 as representative of the series), Tantalum oxide Ta2O5 (0.5 g, 1.13 mmol) was suspended in 30 mL ethanol under stirring. 1 g (12.52 mmol) of Degussa P25 TiO2 powder was suspended in 30 mL ethanol and the suspension then sonicated for 1 h at room temperature. The two suspensions of TiO2 and Nb2O5 or Ta2O5 were mixed under vigorous stirring for 4 hours at room temperature and then dried under vacuum to obtain a powder which was further dried in oven for 8 h at 80°C. The obtained powder, was ground and then calcinated at 500°C for 3 h in the presence of air with a ramp rate of 2°C/min.


## 2.2. Characterization

The crystalline structure of the prepared Ta/TiO2- and Nb/TiO2-mixed oxides photocatalysts was characterized by X-ray powder diffraction (XRD) analysis (RINT Ultima III, Rigaku co., Japan) using Cu Ka radiation at a scan rate of 4 degree/min. The acceleration voltage and the applied current were 40 kV and 40 mA, respectively. The mean size of crystallite (D, nm) was calculated from full-width at half maxima (FWHM) of corresponding X-ray diffraction peaks using Scherrer’s formula D=0.89λ/βcosϕ, whereλ is the wavelength of the X-ray radiation (λ = 1.54056 nm Cu Ka), β is the full-width at half maximum (rad), and ϕ is the reflect angle. The content of anatase wA(%) was determined according to the following equation [24]: wA(%)={IA/(IA+1.265IR)}×100, where IR and IA are the intensities of the diffraction peaks of rutile (110) and anatase (101), respectively, obtained from XRD patterns. X-ray photoelectron spectra (XPS) of the powders were measured using Shimadzu ESCA 750 photoelectron spectrometer with an MgKα 1253.6 ev; the shift of binding energy due to relative surface charging was corrected using the C1s level at 285 eV as an internal standard. The diffuse reflectance UV-Vis spectra were measured with an Ocean Optics high-resolution HR4000 USB spectrometer (HR4000 Ocean Optics Inc.) with an extended wavelength range from 190 to 1100 nm. The microstructures of the samples were observed by a scanning electron microscope, SEM (JSM-5310LV, Jeol Co., Japan).


## 2.3. Photocatalytic Activity Measurement

Solar photocatalytic decolorization and mineralization of Orange II dye was carried out in 300 mL Pyrex glass beaker. All the reactions were carried out according to the following procedure: 0.4 g of photocatalyst (modified or unmodified TiO2) was added to 250 mL of aqueous dye (Orange II) solution (20 mg/L). The dyes solution was mixed with a magnetic stirrer during the course of the experiment. At the beginning, the solution mixture was stirred for 30 min in the dark to ensure establishment of the dye’s equilibrium adsorption/desorption. Then sun light was allowed to irradiate the reaction mixture, and at regular time intervals, samples were taken from the suspension and the change of Orange-II concentration was measured using UV-Vis spectrophotometer (TU-1900 UV spectrometer) at fixed wavelength of 486 nm. For this purpose, the photocatalyst was immediately removed from the sample by filtration, using a 0.4 μm syringe filter. Solar experiments were carried out from 10:00 AM to 5:00 PM during May in Saitama, Japan. During the solar experiments, the UV and the visible ranges (illuminance) of the solar light intensity were measured by the UV radiometer (UVR-2, TOPCON, with UD-36 (310–400 nm/average 365 nm) detector, Tokyo, Japan) and illuminance meter (T-10, Konica Minolta), respectively. The accumulated solar energy was calculated by [25]
(1)Qn=Qn-1+ΔtI(AV),Δt=tn-tn-1.Qn accumulated solar energy per unit of slurry volume (KJL-1), Δtis the time difference between radiation measurements (h), I is the solar light intensity per unit of irradiation surface area measured during time interval Δt (Wm−2), A is the irradiated surface area of the photoreactor (m2), and V is the photoreactor volume (m3).


## 3. Results and Discussion

### 3.1. Characterization of the Prepared Photocatalysts

Figures1(a) and 1(b) show the XRD patterns of Ta/TiO2- and Nb/TiO2-mixed oxides photocatalysts beside the unmodified TiO2 samples. The anatase (101) peak was used to determine the grain size by Scherer’s formula. It is evident that the Nb and Ta contents significantly influenced the particle size (FWHM) and crystallinity (anatase and rutile phases) of the modified samples. It was observed that the XRD patterns of TTa1 and TNb1 samples showed no signals originating the presence of Ta2O5 and Nb2O5 metal oxides, respectively. Actually at these low concentrations (2.9 mol% Ta and 1.8 mol% Nb), Ta and Nb ions are randomly dispersed in the crystallographic sites of anatase structure. Furthermore, the absence of metal peaks could come from their ultra fine dispersion on TiO2 particles as very small clusters or due to very low metal content.(a) XRD results for untreated (pure TiO2) and Ta/TiO2-mixed oxides at different TiO2 : Ta ratios and 500°C calcination temperature for 3 hours. (b) XRD results for untreated (pure TiO2) and Nb/TiO2-mixed oxides at different TiO2 : Nb ratios and 500°C calcination temperature for 3 hours.
(a)(b)Figure1(a) shows the X-ray diffraction patterns for the Ta/TiO2-mixed oxides samples, at high concentrations of 8.3% Ta and 15.3% Ta (TTa2 and TTa3 samples), the results clearly show the presence of peaks corresponding to orthorhombic Ta2O5 (JCPDS 79–1375) [26]. The intensities of those peaks significantly increased with increasing the Ta2O5 concentration mainly due to segregation. Furthermore, increasing the Ta2O5 concentration could not prevent the grain growth, the average crystalline size of unmodified TiO2 (TTa0) increased from 19.50 nm to 21.80 nm for TTa3 (see SEM results). In contrary the results of the grain size analysis of Nb/TiO2-mixed oxide photocatalyst (Figure 1(b)) showed that the grain growth of the unmodified TiO2 (TNb0) have been retarded due to the addition of niobium ion. The crystalline size of TNb1 and TNb3 was 17.79 nm and 18 nm, respectively.Moreover, increasing the Nb2O5 concentration has stabilized the anatase phase, the anatsae content for TNb1, TNb2, and TNb3 samples was about 86%. Our results are consistent with the results of Arbiol et al. [27] who studied the effect of Nb doping in samples synthesized by induced laser pyrolysis. They found that the presence of Nb substitutional ions in the anatase structure hindered the particles growth and transformation from anatase to rutile in TiO2 nanoparticles.The above trend of XRD results is in agreement with the SEM analysis that indicates a decrease of the TNb particle size, as the Nb ion prevents the grain growth [28]. SEM analysis was used to attain morphological structure of the investigated photocatalysts.The SEM images of the Ta/TiO2- and Nb/TiO2-mixed oxides samples besides the unmodified TiO2 sample (TNb1, TTa1, and unmodified TiO2) are shown in Figure 2. The SEM image of unmodified TiO2 (Figure 2(a)) reveals that the powder consists of large micron-scale spherical agglomerates, which consist of tightly packed nanoparticles. For TTa1 sample (Figure 2(b)), some large particles obviously appear. It can be deduced that the appearing large particles may be due to the agglomeration of nanoparticles during the heat treatment. However, TNb1 sample (Figure 2(c)) seems to be relatively homogenous and to contain no large particles. The powder that possesses a sponge-like structure was found to be fine and slightly agglomerate. This structure is beneficial to enhancing the adsorption of reactants [29].SEM photographs showing the untreated TiO2 (Degussa P25) (a) and treated samples after calcinations at 500°C for 4 hours (b) Ta/TiO2-mixed oxides (TTa1), (c) Nb/TiO2-mixed oxides (TNb1).
(a)(b)(c)XPS spectra of the unmodified TiO2-, Nb/TiO2- and Ta/TiO2-mixed oxides powders are shown in Figure 3(a) for Ti 2p, Figure 3(b) for Nb 3d, and Figure 3(c) for Ta 4f. Figure 3(a) shows that the peak position of Ti 2p3/2 corresponds to that of the Ti4+ oxidation state [1]. It appears that the TiO2 phase has been changed due to Nb and Ta species, since the full width at half-maximum (FWHM) for Ti 2p3/2 for the treated and untreated TiO2 samples is not the same. This is in contrast to Atashbar et al. [8] who found no change in TiO2 phase due to Nb species as the FWHM for both the Nb-treated and untreated TiO2 was the same, which could be attributed to the low amount of Nb used in their work (1% wt). Concerning the Nb 3d spectra for Nb/TiO2-mixed oxide (TNb1) shown in Figure 3(b), two peaks are observed at 206.7 and 209.8 eV, the peaks represent the 3d5/2 and 3d3/2 components, respectively. The center of the Nb 3d3/2 peak corresponds to that of Nb5+ oxidation state [8, 30]. Nb5+ species, substituting for Ti4+ in the crystalline lattice, could be a reason for anatase stabilization. Figure 3(c) shows the XPS results for Ta 4f spectra for Ta/TiO2-mixed oxide (TTa1). The peaks at 26.4 eV and 28.1 eV were assigned to Ta5+ [15]. No impurities, such as Ta2O5, were observed for the sample at 1.8 mol% Ta (TTa1), indicating that the Ta ion was substituted at Ti sites. Therefore, it can be concluded that the oxidation state of both tantalum and niobium ions in titania (anatase) is +5. Several authors [31, 32] have illustrated that both Nb5+ and Ta5+ ions substitute for Ti4+ ions in normal lattice sites. In order to maintain the equilibrium of charges, the extrapositive charge due to Nb5+ or Ta5+ may be compensated by the creation of an equivalent amount of Ti3+ ions [33] or by the presence of vacancies in the cation sites [34]. Oxygen vacancies facilitate visible light absorption by generating discrete states about 0.75 eV and 1.18 eV below the conduction band of titanium dioxide [35]. Oxygen vacancies are active electron traps. Since the oxygen defect states lie close to the conduction band of titania, the electrons captured by oxygen defects can be promoted to the surface by visible light absorption.(a) XPS profiles of Ti2p spectra for the untreated TiO2 (Degussa P25) and treated TiO2 photocatalyst after calcination at 500°C for 3 hours in air. (b) Typical Nb 3d spectrum for Nb/TiO2 mixed oxides. (c) Typical Ta 4f spectrum for Ta/TiO2 mixed oxides.
(a)(b)(c)Figure4 shows the UV-Vis absorbance spectra of the untreated TiO2 (pure TiO2), Ta/TiO2-mixed oxides (TTa1 and TTa3), and Nb/Ti-mixed oxides (TNb1 and TNb3). Noticeable shifts of the optical absorption shoulders toward the visible light regions of the solar spectrum were observed for all modified photocatalysts. Notably, this shift towards the longer wavelength originates from the band gap narrowing of TiO2 by Ta and Nb ions; this feature was more evident for (TNb1). The results clearly indicate that visible light absorption of the TiO2 (P25 Degussa) is enhanced by introducing the Ta and Nb ions.Figure 4
UV-vis diffuse reflectance spectra of the Ta/TiO2, Nb/TiO2, and untreated TiO2 at different ratios. The insert Figure shows the band gap estimation.Estimations of the bandgap energies were obtained from the diffuse reflectance spectra of the prepared powders. The relationship between the absorption coefficient(α) and incident photon energy (hv) can be written as α=Bd(hv-Eg)0.5/λ, where Bd is the absorption constant for direct transition [36]. A plot of (Ahv)0.5versus hv from the spectral data is shown in the insert to Figure 4. Extrapolating the linear part of the curve for the photocatalysts gives the band gap values. The linear part of the curve for the untreated TiO2 gives a band gap value of 3.10 eV, which is very close to the commercial Degussa TiO2 (3–3.2 eV) [37, 38]. The estimated band gap energies of the samples from Figure 4 are summarized in Table 2. The maximum band gap reduction was 0.30 eV for TNb1. It has been acknowledged that solar wavelength shows maximum irradiance at the wavelength region of 450–480 nm [39]. Since TNb1 band gap corresponds to this region, it can absorb relatively higher photon flux compared to untreated TiO2. Probably the distortion of the local lattice of TiO2 by Nb5+ and Ta+5 is responsible for the absorption in the visible region and to the shift of the onset of their absorption edge near 400 nm. Previous results (Figure 3) clearly showed that the crystal lattices of the TiO2 powders are locally distorted by incorporating Nb5+ and Ta+5 species into TiO2. These would imply that the Nb and Ta ions can form a new band above the valence band and relatively narrow the band-gap of the photocatalyst, giving rise to the absorption edge in the visible light region (Figure 4). It was found that the solar photocatalytic activity for TNb1 is better than TNb3 (Table 2). Too much of new-generated band-gap structures due to higher Nb or Ta ions concentration could act as recombination centers for electron-hole pairs and consequently reduce the photoactivity of the catalyst.Table 2
Characterization of the prepared samples.
SampleCrystalline size,D (nm)Anatase content (%)Band gap (eV)Decolorization rate,Kap (min−1)Pure TiO219.5080.023.100.0152Nb/TiO2 (TNb1)17.7986.032.800.0364Nb/TiO2 (TNb3)18.0085.702.930.0216Ta/TiO2 (TTa1)20.7475.503.010.0212Ta/TiO2 (TTa3)21.8082.012.980.0192


### 3.2. Solar Photocatalyst Activity

To evaluate the photocatalytic activity of the prepared photocatalysts (TTa1, TTa3, TNb1, and TNb3), under natural solar light irradiation, tests were carried out to decolorize and mineralize Orange II dye in an aqueous suspension at an initial Orange II concentration of 20 mg/L. Figure5 demonstrates the solar photodecolorization and mineralization observed for Orange II in the presence of treated and untreated TiO2 powders. It worth mentioned here that in the dark experiments there is not a definite correlation between the adsorption properties and the activity of the samples, that is, the sponge-like structure sample (TNb1) does not mean the higher the decomposition rate of Orange II dye in the dark. Xie et al. [29] got the same conclusion when comparing the photocatalytic and adsorption properties of their prepared samples.Figure 5
Solar photocatalytic decolorization of Orange II applying treated and untreated TiO2. (Orange II conc. = 10 mg/L, catalyst conc. = 1.8 g/L).During the course of the solar experiments, the total accumulated UV light (350–400 nm) and the total accumulated visible light (400–750 nm) were 0.457 KJ/L and 51.94 KJ/L, respectively.A plot of ln (C/C0) versus t represents approximate linear straight lines, showing the case of the first-order reaction. The slope of the line equals the apparent first-order rate constant (Kap), the estimated Kap values are summarized in Table 2.The activity of the samples was found to be dependent on the metal ions (Nb or Ta) and its amount (1 : 0.1 or 1 : 1 mass ratios). Compared with the untreated TiO2, both Ta/Ti- and Nb/TiO2-mixed oxides photocatalysts exhibit better photocatalytic activity, and obviously Nb/TiO2 sample can more readily photodegrade and mineralized Orange II dye than Ta/TiO2 sample (Table 2). The untreated-TiO2 photocatalytic activity under solar light increased about 140% by treating with Nb at mass ratio of 1 : 0.1 (TNb1); however, it was only about 40% by treating with Ta at mass ratio of 1 : 0.1 (TTa1). Mineralization of the Orange II dye also has been tested under natural solar light (Figure 5). The TNb1 sample significantly enhanced the mineralization rate of Orange II dye (by about 237%). The stability of the catalysts has been tested by using the photocatalysts repeatedly three times (results are not shown). No visible change of the photoactivity has been observed throughout these three runs. One may raise the doubt of whether it is the photocatalyst that plays the key role in decomposing Orange II dye because the dye can absorb visible light itself. If the decomposition of Orange II dye is due to the light absorbance itself, then the efficiency of the decomposition using a different photocatalysts may not vary so much as shown in Figure 5. Orange II dye was not a subject of photolysis, and any change in Orange II dye concentration can be attributed only to the heterogeneous photocatalysis [40].It seems that Nb ions introduce some shallow donor levels below the conduction band edge, which can act as electron traps to retard electron-hole recombination as indicated by band gap reduction in Figure4. Therefore, the lifetime of photogenerated electrons and holes can be increased, which is beneficial to enhancing the photocatalytic efficiency [14]. It was found that the solar photocatalytic activity for TNb1 is better than TNb3 (Table 2). Higher Nb or Ta ions concentration could act as recombination centers for electron-hole pairs and consequently reduce the photoactivity of the catalyst. The excess loading of metal particles may cover active sites on the TiO2 surface thereby reducing photodegradation efficiency [40]. Yu et al. [41] and Zhou et al. [42] pointed out that the photocatalytic activity of the metal dopants (Fe3+ ions) is strongly dependent on the dopant concentration since the metal ion (Fe3+) can serve not only as a mediator of interfacial charge transfer but also as a recombination center. Also the morphology and surface structure (Figure 2) as well as the electronic structure of TiO2 product modified by Nb and Ta ions should play important roles in enhancing photoactivity [43]. An efficient way to enhance the performance of the photocatalyst is to create the hierarchically porous structures in photocatalytic materials [44]. The solar photoactivities of Nb/TiO2 mixed oxide powder (TNb1) are predominantly attributed to an improvement in anatase crystallinity, sponge-like surface structure, low band gap, and low particle size.


## 3.1. Characterization of the Prepared Photocatalysts

Figures1(a) and 1(b) show the XRD patterns of Ta/TiO2- and Nb/TiO2-mixed oxides photocatalysts beside the unmodified TiO2 samples. The anatase (101) peak was used to determine the grain size by Scherer’s formula. It is evident that the Nb and Ta contents significantly influenced the particle size (FWHM) and crystallinity (anatase and rutile phases) of the modified samples. It was observed that the XRD patterns of TTa1 and TNb1 samples showed no signals originating the presence of Ta2O5 and Nb2O5 metal oxides, respectively. Actually at these low concentrations (2.9 mol% Ta and 1.8 mol% Nb), Ta and Nb ions are randomly dispersed in the crystallographic sites of anatase structure. Furthermore, the absence of metal peaks could come from their ultra fine dispersion on TiO2 particles as very small clusters or due to very low metal content.(a) XRD results for untreated (pure TiO2) and Ta/TiO2-mixed oxides at different TiO2 : Ta ratios and 500°C calcination temperature for 3 hours. (b) XRD results for untreated (pure TiO2) and Nb/TiO2-mixed oxides at different TiO2 : Nb ratios and 500°C calcination temperature for 3 hours.
(a)(b)Figure1(a) shows the X-ray diffraction patterns for the Ta/TiO2-mixed oxides samples, at high concentrations of 8.3% Ta and 15.3% Ta (TTa2 and TTa3 samples), the results clearly show the presence of peaks corresponding to orthorhombic Ta2O5 (JCPDS 79–1375) [26]. The intensities of those peaks significantly increased with increasing the Ta2O5 concentration mainly due to segregation. Furthermore, increasing the Ta2O5 concentration could not prevent the grain growth, the average crystalline size of unmodified TiO2 (TTa0) increased from 19.50 nm to 21.80 nm for TTa3 (see SEM results). In contrary the results of the grain size analysis of Nb/TiO2-mixed oxide photocatalyst (Figure 1(b)) showed that the grain growth of the unmodified TiO2 (TNb0) have been retarded due to the addition of niobium ion. The crystalline size of TNb1 and TNb3 was 17.79 nm and 18 nm, respectively.Moreover, increasing the Nb2O5 concentration has stabilized the anatase phase, the anatsae content for TNb1, TNb2, and TNb3 samples was about 86%. Our results are consistent with the results of Arbiol et al. [27] who studied the effect of Nb doping in samples synthesized by induced laser pyrolysis. They found that the presence of Nb substitutional ions in the anatase structure hindered the particles growth and transformation from anatase to rutile in TiO2 nanoparticles.The above trend of XRD results is in agreement with the SEM analysis that indicates a decrease of the TNb particle size, as the Nb ion prevents the grain growth [28]. SEM analysis was used to attain morphological structure of the investigated photocatalysts.The SEM images of the Ta/TiO2- and Nb/TiO2-mixed oxides samples besides the unmodified TiO2 sample (TNb1, TTa1, and unmodified TiO2) are shown in Figure 2. The SEM image of unmodified TiO2 (Figure 2(a)) reveals that the powder consists of large micron-scale spherical agglomerates, which consist of tightly packed nanoparticles. For TTa1 sample (Figure 2(b)), some large particles obviously appear. It can be deduced that the appearing large particles may be due to the agglomeration of nanoparticles during the heat treatment. However, TNb1 sample (Figure 2(c)) seems to be relatively homogenous and to contain no large particles. The powder that possesses a sponge-like structure was found to be fine and slightly agglomerate. This structure is beneficial to enhancing the adsorption of reactants [29].SEM photographs showing the untreated TiO2 (Degussa P25) (a) and treated samples after calcinations at 500°C for 4 hours (b) Ta/TiO2-mixed oxides (TTa1), (c) Nb/TiO2-mixed oxides (TNb1).
(a)(b)(c)XPS spectra of the unmodified TiO2-, Nb/TiO2- and Ta/TiO2-mixed oxides powders are shown in Figure 3(a) for Ti 2p, Figure 3(b) for Nb 3d, and Figure 3(c) for Ta 4f. Figure 3(a) shows that the peak position of Ti 2p3/2 corresponds to that of the Ti4+ oxidation state [1]. It appears that the TiO2 phase has been changed due to Nb and Ta species, since the full width at half-maximum (FWHM) for Ti 2p3/2 for the treated and untreated TiO2 samples is not the same. This is in contrast to Atashbar et al. [8] who found no change in TiO2 phase due to Nb species as the FWHM for both the Nb-treated and untreated TiO2 was the same, which could be attributed to the low amount of Nb used in their work (1% wt). Concerning the Nb 3d spectra for Nb/TiO2-mixed oxide (TNb1) shown in Figure 3(b), two peaks are observed at 206.7 and 209.8 eV, the peaks represent the 3d5/2 and 3d3/2 components, respectively. The center of the Nb 3d3/2 peak corresponds to that of Nb5+ oxidation state [8, 30]. Nb5+ species, substituting for Ti4+ in the crystalline lattice, could be a reason for anatase stabilization. Figure 3(c) shows the XPS results for Ta 4f spectra for Ta/TiO2-mixed oxide (TTa1). The peaks at 26.4 eV and 28.1 eV were assigned to Ta5+ [15]. No impurities, such as Ta2O5, were observed for the sample at 1.8 mol% Ta (TTa1), indicating that the Ta ion was substituted at Ti sites. Therefore, it can be concluded that the oxidation state of both tantalum and niobium ions in titania (anatase) is +5. Several authors [31, 32] have illustrated that both Nb5+ and Ta5+ ions substitute for Ti4+ ions in normal lattice sites. In order to maintain the equilibrium of charges, the extrapositive charge due to Nb5+ or Ta5+ may be compensated by the creation of an equivalent amount of Ti3+ ions [33] or by the presence of vacancies in the cation sites [34]. Oxygen vacancies facilitate visible light absorption by generating discrete states about 0.75 eV and 1.18 eV below the conduction band of titanium dioxide [35]. Oxygen vacancies are active electron traps. Since the oxygen defect states lie close to the conduction band of titania, the electrons captured by oxygen defects can be promoted to the surface by visible light absorption.(a) XPS profiles of Ti2p spectra for the untreated TiO2 (Degussa P25) and treated TiO2 photocatalyst after calcination at 500°C for 3 hours in air. (b) Typical Nb 3d spectrum for Nb/TiO2 mixed oxides. (c) Typical Ta 4f spectrum for Ta/TiO2 mixed oxides.
(a)(b)(c)Figure4 shows the UV-Vis absorbance spectra of the untreated TiO2 (pure TiO2), Ta/TiO2-mixed oxides (TTa1 and TTa3), and Nb/Ti-mixed oxides (TNb1 and TNb3). Noticeable shifts of the optical absorption shoulders toward the visible light regions of the solar spectrum were observed for all modified photocatalysts. Notably, this shift towards the longer wavelength originates from the band gap narrowing of TiO2 by Ta and Nb ions; this feature was more evident for (TNb1). The results clearly indicate that visible light absorption of the TiO2 (P25 Degussa) is enhanced by introducing the Ta and Nb ions.Figure 4
UV-vis diffuse reflectance spectra of the Ta/TiO2, Nb/TiO2, and untreated TiO2 at different ratios. The insert Figure shows the band gap estimation.Estimations of the bandgap energies were obtained from the diffuse reflectance spectra of the prepared powders. The relationship between the absorption coefficient(α) and incident photon energy (hv) can be written as α=Bd(hv-Eg)0.5/λ, where Bd is the absorption constant for direct transition [36]. A plot of (Ahv)0.5versus hv from the spectral data is shown in the insert to Figure 4. Extrapolating the linear part of the curve for the photocatalysts gives the band gap values. The linear part of the curve for the untreated TiO2 gives a band gap value of 3.10 eV, which is very close to the commercial Degussa TiO2 (3–3.2 eV) [37, 38]. The estimated band gap energies of the samples from Figure 4 are summarized in Table 2. The maximum band gap reduction was 0.30 eV for TNb1. It has been acknowledged that solar wavelength shows maximum irradiance at the wavelength region of 450–480 nm [39]. Since TNb1 band gap corresponds to this region, it can absorb relatively higher photon flux compared to untreated TiO2. Probably the distortion of the local lattice of TiO2 by Nb5+ and Ta+5 is responsible for the absorption in the visible region and to the shift of the onset of their absorption edge near 400 nm. Previous results (Figure 3) clearly showed that the crystal lattices of the TiO2 powders are locally distorted by incorporating Nb5+ and Ta+5 species into TiO2. These would imply that the Nb and Ta ions can form a new band above the valence band and relatively narrow the band-gap of the photocatalyst, giving rise to the absorption edge in the visible light region (Figure 4). It was found that the solar photocatalytic activity for TNb1 is better than TNb3 (Table 2). Too much of new-generated band-gap structures due to higher Nb or Ta ions concentration could act as recombination centers for electron-hole pairs and consequently reduce the photoactivity of the catalyst.Table 2
Characterization of the prepared samples.
SampleCrystalline size,D (nm)Anatase content (%)Band gap (eV)Decolorization rate,Kap (min−1)Pure TiO219.5080.023.100.0152Nb/TiO2 (TNb1)17.7986.032.800.0364Nb/TiO2 (TNb3)18.0085.702.930.0216Ta/TiO2 (TTa1)20.7475.503.010.0212Ta/TiO2 (TTa3)21.8082.012.980.0192


## 3.2. Solar Photocatalyst Activity

To evaluate the photocatalytic activity of the prepared photocatalysts (TTa1, TTa3, TNb1, and TNb3), under natural solar light irradiation, tests were carried out to decolorize and mineralize Orange II dye in an aqueous suspension at an initial Orange II concentration of 20 mg/L. Figure5 demonstrates the solar photodecolorization and mineralization observed for Orange II in the presence of treated and untreated TiO2 powders. It worth mentioned here that in the dark experiments there is not a definite correlation between the adsorption properties and the activity of the samples, that is, the sponge-like structure sample (TNb1) does not mean the higher the decomposition rate of Orange II dye in the dark. Xie et al. [29] got the same conclusion when comparing the photocatalytic and adsorption properties of their prepared samples.Figure 5
Solar photocatalytic decolorization of Orange II applying treated and untreated TiO2. (Orange II conc. = 10 mg/L, catalyst conc. = 1.8 g/L).During the course of the solar experiments, the total accumulated UV light (350–400 nm) and the total accumulated visible light (400–750 nm) were 0.457 KJ/L and 51.94 KJ/L, respectively.A plot of ln (C/C0) versus t represents approximate linear straight lines, showing the case of the first-order reaction. The slope of the line equals the apparent first-order rate constant (Kap), the estimated Kap values are summarized in Table 2.The activity of the samples was found to be dependent on the metal ions (Nb or Ta) and its amount (1 : 0.1 or 1 : 1 mass ratios). Compared with the untreated TiO2, both Ta/Ti- and Nb/TiO2-mixed oxides photocatalysts exhibit better photocatalytic activity, and obviously Nb/TiO2 sample can more readily photodegrade and mineralized Orange II dye than Ta/TiO2 sample (Table 2). The untreated-TiO2 photocatalytic activity under solar light increased about 140% by treating with Nb at mass ratio of 1 : 0.1 (TNb1); however, it was only about 40% by treating with Ta at mass ratio of 1 : 0.1 (TTa1). Mineralization of the Orange II dye also has been tested under natural solar light (Figure 5). The TNb1 sample significantly enhanced the mineralization rate of Orange II dye (by about 237%). The stability of the catalysts has been tested by using the photocatalysts repeatedly three times (results are not shown). No visible change of the photoactivity has been observed throughout these three runs. One may raise the doubt of whether it is the photocatalyst that plays the key role in decomposing Orange II dye because the dye can absorb visible light itself. If the decomposition of Orange II dye is due to the light absorbance itself, then the efficiency of the decomposition using a different photocatalysts may not vary so much as shown in Figure 5. Orange II dye was not a subject of photolysis, and any change in Orange II dye concentration can be attributed only to the heterogeneous photocatalysis [40].It seems that Nb ions introduce some shallow donor levels below the conduction band edge, which can act as electron traps to retard electron-hole recombination as indicated by band gap reduction in Figure4. Therefore, the lifetime of photogenerated electrons and holes can be increased, which is beneficial to enhancing the photocatalytic efficiency [14]. It was found that the solar photocatalytic activity for TNb1 is better than TNb3 (Table 2). Higher Nb or Ta ions concentration could act as recombination centers for electron-hole pairs and consequently reduce the photoactivity of the catalyst. The excess loading of metal particles may cover active sites on the TiO2 surface thereby reducing photodegradation efficiency [40]. Yu et al. [41] and Zhou et al. [42] pointed out that the photocatalytic activity of the metal dopants (Fe3+ ions) is strongly dependent on the dopant concentration since the metal ion (Fe3+) can serve not only as a mediator of interfacial charge transfer but also as a recombination center. Also the morphology and surface structure (Figure 2) as well as the electronic structure of TiO2 product modified by Nb and Ta ions should play important roles in enhancing photoactivity [43]. An efficient way to enhance the performance of the photocatalyst is to create the hierarchically porous structures in photocatalytic materials [44]. The solar photoactivities of Nb/TiO2 mixed oxide powder (TNb1) are predominantly attributed to an improvement in anatase crystallinity, sponge-like surface structure, low band gap, and low particle size.


## 4. Conclusions

The commercially available TiO2 photocatalyst (Degussa P25) has been treated with niobium (Nb) and tantalum (Ta) ions, employing simple impregnation method at room temperature. The prepared Nb/TiO2- and Ta/TiO2-mixed oxide powders showed higher activity than the untreated TiO2 under natural solar light. The maximum activity was observed for Nb/TiO2 sample at mass ratio of 1 : 0.1 (TNb1). Comparing with the untreated TiO2, the solar decolorization and mineralization rates improved by 140% and 237%, respectively, and the band gap reduced to 2.80 eV and with a stabilized anatase phase (86% anatsa content). XPS results showed that the oxidation state of Ta and Nb ions in titania is +5 and leads to the substitution of Ti4+ by Nb5+ and Ta5+ in Nb/TiO2 and Ta/TiO2 samples, respectively. The results suggest that the crystal lattices of Nb/TiO2 powder are locally distorted by incorporating Nb5+ species into TiO2, forming a new band energy structure, which is responsible for the absorption in the visible region. The Nb/TiO2 sample (TNb1) that is characterized by the smallest crystalline size (17.79 nm), not only stabilizes the anatase phase but also prevents the grain growth to some extent, which is important to achieve high solar photoactivity, while Ta/TiO2 sample did not. Therefore, Nb/TiO2-mixed oxide is more suitable than Ta/TiO2 one. The metal ions (Nb or Ta) segregation become significant in the treated samples of higher than 1.8% Ta and 2.9% Nb. The solar photoactivities of Nb/TiO2 powder are predominantly attributed to an improvement in anatase crystallinity, low band gap, and low particle size.



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