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Study of new states in visible light active W, N co-doped TiO2 photo catalyst
Materials Research Bulletin 47 (2012) 3083–3089
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Study of new states in visible light active W, N co-doped TiO2 photo catalyst Ahmed Khan Leghari Sajjad, Sajjad Shamaila *, Jinlong Zhang Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 December 2011 Received in revised form 26 July 2012 Accepted 11 August 2012 Available online 18 August 2012
The visible light efficient W, N co-doped TiO2 photo catalysts are prepared by sol–gel method. New linkages of N, W and O are formed as N–Ti–O, N–W–O, Ti–O–N and W–O–N. Electron paramagnetic resonance illustrates the presence of oxygen vacancies in W, N co-doped TiO2 acting as trapping agencies for electrons to produce active species. X-ray photoelectron spectroscopy confirms the presence of new energy states. New linkages and oxygen vacancies are proved to be the main cause for the improved photo catalytic performances. W, N co-doped TiO2 has new energy states which narrow the band gap effectively. W, N co-doped TiO2 is thermally stable and retains its anatase phase up to 900 8C. 4.5% W, N co-doped TiO2 showed superior activity for the degradation of Rhodamine B and 2,4-dichlorophenol as compared to pure titania, Degussa P-25, traditional N-doped TiO2 and pure WO3. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Sol–gel chemistry D. Defects D. Electronic paramagnetic resonance
1. Introduction TiO2 photocatalysis has attracted extensive attention as a promising technique for the degradation of pollutants due to high oxidative power, photo stability and nontoxicity [1]. Generally, TiO2 is only sensitive to UV light due to its large band gap (3.2 eV) [2]. In recent years, great efforts have been made to develop TiO2-based photo catalysts sensitive to visible light in order to make use of solar energy more efficiently in practical applications such as surface modification [3], metal or nonmetal ion doping [4,5], generation of oxygen vacancies [6], combination with other semiconductors [7,8]. Among these strategies, TiO2 doped with nonmetals such as N and C has been considered one of the most promising ways to develop TiO2 photo catalysts sensitive to visible light [9,10]. The visible light sensitive TiO2 photo catalyst is prepared by a hydrolytic process [11]. N-doped TiO2 causes the photo catalytic decomposition of acetaldehyde to CO2 under visible light [12]. Nitrogen doped TiO2 exhibits higher photo catalytic activity for degradation of methylene blue in aqueous solution compared to Degussa P25 under visible light [13]. WO3 coupling has been widely used to improve the photo electrochemical and photo catalytic performance of TiO2 because WO3 is more acidic than TiO2 and can serve as an electron acceptor [14,15]. In case of W (VI), the cation tends to
* Corresponding author. Tel.: +86 92514444024; fax: +86 92514444024. E-mail address: [email protected] (S. Shamaila). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.08.032
migrate toward the surface shifting the isoelectric point to lower values in comparison to pristine TiO2 and reduces the charge recombination [16,17]. Reports on metal and non-metal codoped TiO2 catalysts are seldom. It is suggested that the formation of paramagnetic N species in metal and nitrogen doped samples at interstitial positions in TiO2 lattice was responsible for the visible light response of the catalyst in degradation of acetaldehyde [18]. N-doped TiO2 coupled with tungsten oxide samples were prepared using different methods. However, the preparation methods were too much complicated [19,20]. Previously, iron and nitrogen co-doped TiO2 was synthesized using homogeneous precipitation and then followed with hydrothermal process in the presence of triethylamine [21]. Hao and Zhang prepared iron (III) and nitrogen co-doped mesoporous TiO2 by modified sol–gel method and the nitrogen containing surfactant (dodecylamine) was introduced as a structure directing agent as well as a nitrogen dopant [22]. In this study, W, N co-doped TiO2 photocatalysts are synthesized by simple sol–gel method. New linkages of N–Ti–O, N–W–O, Ti–O–N and W–O–N are responsible for the decrease of band gap energy to enhance its visible light absorption resulting in higher degradation rates. Oxygen vacancies generated during co-doping in W, N codoped TiO2 samples act as trapping sites which effectively decrease recombination rate of the photo induced electrons and holes, thus increase the photo-oxidation efficiency. 4.5% N, W co-doped TiO2 showed far superior activity as compared to P-25, pure TiO2, traditional N-doped TiO2 and pure WO3 for the degradation of RhB and 2,4-DCP in visible and ultraviolet irradiations.
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2. Experimental
2.4. Measurement of photocatalytic activities
2.1. Materials
The photo catalytic performance of W, N-TiO2 nanocomposites was evaluated in the degradation of RhB and 2,4-DCP. The first one is representative of dyes while 2,4-DCP is a highly toxic pollutant. We studied the photo catalytic degradation processes on exposure to visible and UV light. The photo catalytic degradation was carried out in a 100 mL quartz photochemical reactor. The initial concentration of RhB and 2,4-DCP in a quartz reaction vessel was fixed at 20 mgL1 with a catalyst loading of 1.1 g L1. A 1000 W halogen lamp was used as light source and the light from the lamp included beams from ultraviolet and visible light regions. The short-wavelength components (l < 420 nm) of light were cut off using a cut-off glass filter for visible photoreaction. The mercury lamp of 300 W is used for the UV light reaction. During the reaction, a water-cooling system cooled the water-jacketed photochemical reactor to maintain the solution at room temperature. The distance between the lamp and the center of quartz tube was 10 cm. Prior to illumination, reaction mixture was sonicated for 20 min to homogenize and the suspension was magnetically stirred in darkness for 30 min to establish adsorption–desorption equilibrium at room temperature. During irradiation, stirring was maintained to keep the mixture in suspension. At regular intervals, samples were withdrawn and centrifuged to separate photo catalyst for analysis. Then filtered through a 0.22 mm millipore filter to remove the photo catalyst. The photo activities for RhB and 2,4-DCP in dark in the presence of the photo catalyst and under visible light irradiation in the absence of the photo catalyst were also evaluated. The extent of RhB and 2,4-DCP decomposition was determined by measuring the absorbance value at approximately 552 and 283 nm, respectively. The measurements were repeated for the catalyst three times and the experimental error was found to be within 3%.
Tetra butyl titanate (TBT) and acetic acid were used as received. Ammonium tungstate, urea and absolute alcohol were of analytical grade received from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China and used without any further purification. Pure WO3 was supplied from Aldrich. 2.2. Preparation 17 mL TBT was added to the mixture of 120 mL absolute ethanol, 15 mL acetic acid and 5 mL doubly distilled water to form the sol. After aging of sol for 48 h, different additions of ammonium tungstate aqueous solution were dripped to the sol under vigorous stirring according to the required amount of ammonium tungstate until the gel formed. The obtained gel was aged, grinded and treated in vacuum at specific temperature. The samples were calcined at different temperatures. Pure TiO2 was prepared by same procedure without addition of ammonium tungstate. For comparison, nitrogen doped TiO2 was also prepared using urea and designated as traditional N-doped TiO2. 2.3. Catalyst characterization UV–vis diffuse reflectance spectra (DRS) were obtained with a Scan UV-vis-NIR spectrophotometer (Varian Cary 500) equipped with an integrated sphere assembly using BaSO4 as a reflectance sample. X-ray diffraction (XRD) measurements were carried out to investigate the crystallographic properties with a Rigaku D/Max 2550 VB/PC apparatus (Cu Ka1 radiation, l = 1.54056 A˚) at room temperature operated at 40 kV and 100 mA. Diffraction patterns were recorded in the angular range of 20–808. The Scherrer equation was applied to estimate the average crystallite sizes of TiO2 samples:
3. Results and discussion 3.1. Crystal structures
Kl D¼ b cos u where b is the half-height width of the diffraction peak of anatase (101), K = 0.89 is a coefficient, u is the diffraction angle, and l is the X-ray wavelength corresponding to the Cu Ka radiation. D is the average crystallite size of the powder sample in nanometer. Fourier transform infra-red (FT-IR) spectra were recorded by employing a Nicolet 740 FT-IR spectrometer equipped with a TGS detector and a KBr beam splitter. Raman spectra of the sample were recorded by Renishaw inVia Raman spectrometer at room temperature with the excitation wavelength of 514.6 nm. The surface morphologies and elemental mapping were observed by scanning electron microscopy (SEM) and energy dispersive X-ray analyses (EDXA), respectively. The SBET of the samples were determined through nitrogen physical adsorption at 77 K (Micromeritics ASAP 2010). All the samples were degassed at 473 K before the measurement. To investigate the chemical states of the photo catalysts, X-ray photoelectron spectra (XPS) were recorded with PerkinElmer PHI 5000C ESCA system with Al Ka radiation operated at 250 W. The shift of binding energy due to relative surface charging was corrected using the C 1s level at 284.6 eV as an internal standard. Electron paramagnetic resonance (EPR) measurements were recorded at room temperature on a Bruker EMX-8/2.7 with the microwave frequency of 9.85 GHz and power of 6.35 mW. UV–vis absorption spectra of the samples were recorded on Cary 100 UV-vis spectrophotometer.
XRD patterns of pure TiO2 and W, N co-doped TiO2 samples calcined at 500 8C are shown in Fig. 1. Pure TiO2 and W, N co-doped TiO2 samples display crystallized anatase phase which correspond to characteristic diffraction peaks. The intensity of the anatase (1 0 1) peak at 25.38 gradually decreased and its peak width became broader with an increase in dopant. Obviously, the incorporation of dopant into the lattice of TiO2 hindered the crystallization resulting peak width broadening. It is worth noting
Fig. 1. XRD patterns of pure TiO2 and doped samples (a) TiO2, (b) 1.5% W, N-TiO2, (c) 3.0% W, N-TiO2, (d) 4.5% W, N-TiO2 and (e) 6.0% W, N-TiO2.
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that no lower intensity peak of dopants could be observed in addition to the anatase peaks up to 4.5% W, N co-doped TiO2. We can propose that dopant ions may be incorporated into TiO2 lattice which replaced titanium ions or located at interstitial sites. Some of the researcher studied that the concentration of dopants is too low for the XRD to reveal [19,16,23]. In contrast, reflections in addition to the anatase phase are seen in XRD pattern of nanocomposites at higher 6.0% W, N-TiO2. The excess dopant does not enter into the crystal lattices of TiO2 and is uniformly dispersed on TiO2 or covered the surface of TiO2. The particle size of the samples is in the range of 11–17 nm using Scherrer equation. 3.2. Thermal stability The presence of dopant can usually retard the crystal growth and phase transformation from anatase to rutile. Thus doping can enhance the thermal stability of the anatase phase [23]. W–N [21] and B–N [24] co-doped TiO2 have been reported to possess small crystallite sizes and high anatase to rutile phase transformation temperatures with respect to single N-doped TiO2. Fig. S1 shows that 4.5% W, N co-doped TiO2 has larger thermal stability for anatase form up to 900 8C and rutile form appears at 1000 8C. The addition of tungsten increases thermal stability and stabilizes the anatase phase [25] while N has little effect [26]. In fact, rutile form predominates at 800 8C in pure TiO2. The doping quantity of 4.5% W, N could affect the diffraction pattern and changed the particle size. Similarly Ce–N [27] and P–N [26] co-doped TiO2 have been reported to possess small crystallite sizes and high anatase to rutile phase transformation temperatures with respect to solely N doped TiO2. The textural properties are changed with the increase in temperature as shown in Table S1. 3.3. UV–vis diffuse reflectance spectroscopy Fig. 2 shows UV–visible diffuse reflectance spectra of pure TiO2, traditional N-doped TiO2, pure WO3, 3.0 and 4.5% W, N co-doped TiO2. Pure TiO2 exhibits only a strong absorption in ultraviolet region which attributes to the band–band transition (Fig. 2a). Fig. 2b shows diffuse reflectance spectrum of traditional N-doped TiO2 which decreases the band gap resulting into red shift. In Ndoped TiO2, oxygen vacancies are the cause for visible light response [28]. The valence band of N-TiO2 consists of N2p and O2p, that is, only donor energy states exist in the band of N-TiO2 [29]. The absorption onset wavelengths of pure WO3 powder are in the range of 450–480 nm (Fig. 2c). In Fig. 2d, 3.0% W, N co-doped TiO2 shows an additional broad absorption that extends to visible
Fig. 3. (a) EDS of 4.5% W, N-TiO2 and (b) SEM image of 4.5% W, N-TiO2.
region in the range of 600–700 nm. However, 4.5% W, N co-doped TiO2 shows an additional broad absorption that extends throughout the entire visible region (Fig. 2e). Besides the contribution of N species to visible light absorption, W could form dopant energy level within the band gap of TiO2. The concentration of dopants also influences markedly the visible absorption and intensity of the band. The absorption intensity also enhances with the increase of doped W and N concentration. The new linkages N–Ti–O, N–W–O, Ti–O–N and W–O–N and oxygen vacancies formed in W, N-TiO2 act as the additional impurity states or defect energy levels which contribute to the enhanced visible light absorption. 3.4. Energy dispersive X-ray spectra and scanning electron microscopy Energy dispersive X-ray analysis (EDXA) is carried out to determine the presence of elements. The EDX spectrum of W, N codoped TiO2 sample is shown in Fig. 3a. The peaks of W, N, Ti and O can be clearly seen. The presence of signal of Au can be ascribed to Au grid. Fig. 3b shows the SEM image of 4.5% W, N co-doped TiO2. It depicts the loose agglomerates with significant quantity of interparticle voids. 3.5. Fourier transforms infra-red spectra
Fig. 2. DRS of pure TiO2 and different samples (a) pure TiO2, (b) traditional N-doped TiO2, (c) pure WO3, (d) 3.0% W, N-TiO2 and (e) 4.5% W, N-TiO2.
The FT-IR spectra of pure TiO2, 1.5, 3.0 and 4.5% W, N co-doped TiO2 samples are shown in Fig. 4. The intensive and broad band at 550–635 cm1 is ascribed to the stretching vibrations of Ti–O bond [30]. The deformation vibrations at 1600–1630 cm1 (H–O–H) are evidence of large amount of water molecules absorbed on the surface of the catalyst [31]. The presence of large amount of
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Fig. 4. FTIR of pure and different % of doped samples (a) pure TiO2, (b) 1.5% W, NTiO2, (c) 3.0% W, N-TiO2 and (d) 4.5% W, N-TiO2.
hydroxyl groups is beneficial for the photo catalytic process. The band at 2360 cm1 is the characteristic of stretching vibrations of N-bond [32] while the presence of band at 1396 cm1 is an evidence of the absorbed molecular oxygen [30]. The broad bands between 500 and 1000 cm1 are attributed to the framework vibrations of WO3 [33]. The new peak at 1010 cm1 in W, N codoped TiO2 (Fig. 4b) can be attributed to the new linkages of W and N which is absent in pure TiO2 (Fig. 4a). 3.6. Raman spectra The Raman spectra of pure TiO2 and different W, N-TiO2 samples are shown in Fig. S2. The anatase bands observed at 397.0, 516.0 and 638.0 cm1 are assigned to the A1g, B2g and Eg vibrational modes of TiO2, respectively [34]. With an increase in incorporated W and N, the bands from the anatase phase are gradually decreased in intensity and become broader. This is ascribed to the lower crystallinity of anatase phase in consistent with the results of XRD. The peaks at 965–975 cm1 are attributed to the symmetric W5 5O stretching mode of tungsten oxide species linked with nitrogen (Fig. S2b and c). This peak is absent in pure TiO2 as shown in Fig. S2a. These results favorably correspond with the reports [20,35]. 3.7. XPS spectra XPS survey spectra for pure TiO2 and 4.5% W, N co-doped TiO2 are shown in Fig. 5. XPS analysis indicates that W, N co-doped TiO2 samples contain Ti, N, W, O and trace amounts of carbon which originates from the residual carbon in the composite and the adventitious hydrocarbon in the XPS instrument itself. The binding energies (B.E) of Ti 2p3/2 and 2p1/2 of pure titania are located at 458.1 and 463.7 eV, respectively, as shown in Fig. 5a which are attributed to Ti4+ [36]. W, N co-doped TiO2 shows the binding energies (B.E) of Ti 2p3/2 and 2p1/2 located at 458.3 and 464.0 eV. There is slight shifting of binding energy of Ti 2p due to the effect of dopant into the lattice of TiO2. In Fig. 5b, the O 1s band of pure TiO2 shows the binding energy at 529.6 eV while W, N co-doped TiO2 shows the binding energy at 529.8 eV. The peaks at 529.6 and 529.8 eV contain contributions from both the Ti–O and W–O, respectively, having almost similar binding energies. In the present work, energy levels of tungsten are measured in W4d5/2 regions as shown in Fig. 5c. The binding energy of W4d5/2 corresponds to 246.7 eV in our experimental results. WO3 shows typical W4d5/2 and W4d3/2 at 247.8 eV and 260.5 eV [20], respectively. The W4d5/2 peaks of W, N-TiO2 experience the shift to lower binding energy as compared to that of W4d5/2 of pure WO3 which indicates higher
electron density of W atoms in W, N co-doped TiO2 sample. The peaks of N 1s and W 4d shift in opposite directions that further suggest the new linkage of N–W in W, N co-doped TiO2 samples. As shown in Fig. 5d, N 1s peak is broader which ranges from 396 to 404 eV in W, N-TiO2 co-doped samples. According to curve fit data, two peaks are observed at 400.4 eV and 401.6 eV binding energies. The peak at 400.4 eV can be attributed to anionic N– in O– Ti–N or O–W–N linkages [37]. The electro negativity of nitrogen doped into TiO2 lattice is lower than oxygen leading to the reduction of electron density on nitrogen due to presence of more oxidized W species. Therefore, the peak at 400.4 eV in our results is higher than that of Ti–N appearing at 396–399 eV [9,38]. The presumed reason of slight shifting of this peak can be attributed to nitrogen replaces the oxygen in the crystal lattice of TiO2. This also confirms the formation of N–Ti–O bonds or the interstitial nitrogen bound to one lattice oxygen and forms the N–O species. Another peak at 401.6 eV is attributed to the formation of Ti–O–N and W– O–N bonds which further confirmed by curve fit data of O 1s as shown in Fig. 5e. There are two peaks after curve fitting. The peak at 529.8 eV is attributed to O1s in Ti–O or W–O linkages. Other peak at 531.6 eV is attributed to the presence of –O–N and –O–N bonds [20,39,40]. Some researchers suggest that the peak is associated with hydroxyl groups [41]. Consequently, the presence of Ti–O–N suggests that peak at 401.0 eV for N1s region of N-TiO2 should be attributed to the oxidized nitrogen of Ti–O–N or W–O–N. It can be assumed that N1s binding energy may vary from case to case when the TiO2xNx is prepared using different chemical methods [39]. From the above observations, it can be concluded that the states of nitrogen and oxygen in W, N co-doped TiO2 may be various and coexist in the forms of different new linkages as N– W–O, N–Ti–O, Ti–O–N and W–O–N in our case. 3.8. EPR spectra EPR spectra of pure and 4.5% W, N-TiO2 samples are shown in Fig. 6 recorded at 298 K. Pure TiO2 shows no significant signal (Fig. 6a). 4.5% W, N co-doped TiO2 sample (Fig. 6b) shows a new significant signal due to shallow donor state at g = 2.002. This result suggests that co-doping favors the formation of oxygen vacancies. Nakamura et al. [42] reported that the symmetrical and sharp EPR signal at g = 2.004 detected on plasma treated TiO2 arose from the electron trapped on the oxygen vacancy. Serwicka et al. [43] observed a sharp signal at g = 2.003 on the vacuum reduced TiO2 at 673–773 K. They attributed this signal to a bulk defect, probably an electron trapped on an oxygen vacancy. Interestingly, similar signal (g = 2.004) was also found in N-doped TiO2 by Feng et al. [44]. Serpone and co-workers assigned the signal at g = 2.003–2.005 to the one electron trapped on the oxygen vacancy or referred to as an F center vacancy [45]. It is reported that F center vacancy located below the conduction band edge of TiO2 results in reduced TiOx and anion doped TiO2 photo catalyst are responsive to visible light [45]. Oxygen vacancies contribute mainly for the optical absorption in this region [46]. There is only one signal with g = 2.002 in 4.5% W, N co-doped TiO2 sample which attributes to the electron trapped on the oxygen vacancy (F centers). It is reasonable to assume that F center vacancy actually exists in the W, N modified TiO2 and existence of oxygen vacancy results in the sensitivity of photo catalyst to visible light. In Fig. 6c, the intensity of EPR signal peak increases obviously after visible light illumination which indicates the existence of free oxygen vacancy due to its trapping of photo-excited electron. 3.9. Photo catalytic degradation of Rhodamine B and 2,4-DCP The photo degradation of RhB and 2,4-DCP is employed to evaluate the photo catalytic activities under visible and UV
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Fig. 5. XPS spectra of pure TiO2 and 4.5% W, N-TiO2 samples (a) Ti 2p, (b) O 1s, (c) W4d, (d) curve fit of N 1s 4.5% W, N-TiO2, (e) curve fit of O 1s of 4.5% W, N-TiO2 and (f) full region XPS survey of 4.5% W, N co-doped TiO2.
irradiations. 4.5% W, N-TiO2 shows 100% photo degradation of RhB and 70.0% 2,4-DCP in visible light in 150 min. W, N co-doped TiO2 samples are more photoactive than bare TiO2, Degussa P-25, traditional N-doped titania and pure WO3 as shown in Fig. 7. The expanded photo activity under visible light is due to W and N codoping and oxygen vacancies. The N–Ti–O or N–W–O linkages forms new energy bands which induce the narrowing of band gap of TiO2. The metal and non metal doped TiO2 decrease the recombination rate of excited electrons and holes. It is proposed that oxygen vacancies are the major cause of visible light response.
The doped W ions with high positive charge act as an accepter and favors to separate photo excited electron/hole pairs [47]. W (VI) species in the present W, N-TiO2 catalysts are also considered to act as trapping site by accepting the photo excited electrons from the TiO2 valence band and then generating W (V). Nitrogen creates the energy levels above the valence band of TiO2. In brief, these two processes co-operated to improve the efficiency of absorbing visible light and form photo induced electrons and holes which both enhance the W, N-TiO2 photo catalytic activity under visible light. For WO3, theoretically, it can be excited by photons with
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Fig. 6. EPR spectra of different samples (a) pure TiO2, (b) 4.5% W, N co-doped TiO2 and (c) 4.5% W, N co-doped TiO2 after visible light illumination of 15 min.
wavelength under 443 nm. But it shows only low photo catalytic activity under visible light because of high charge recombination. Traditional N-doped TiO2 shows higher photo activity as compared to pure TiO2, pure WO3 and Degussa P-25 due to the extension of visible light response to some extent. N-doped TiO2 is responsible for the decrease of band gap [19]. In W, N co-doped TiO2, visible light response becomes stronger with increase of tungsten doping and the generation of linkage of N–W–O and N–Ti–O. It is well known that WO3 retains much higher Lewis surface acidity than TiO2 and has higher affinity for chemical species having unpaired electrons. Therefore, it is much easier for nitrogen species to coordinate with W dispersed on the surface than TiO2 surface itself to form N–W–O linkages as observed in our XPS data which also contributes to visible light responses. When the amount of dopant reaches to 6.0%, the photo catalytic activity of the W, N co-doped TiO2 decrease even though the visible light absorption still increase. It may be due to the charge transfer of the composite catalysts to O2 or target molecules could be retarded with too much dopant. Fig. S3(a and b) shows the degradation of RhB and 2,4-DCP in UV light, respectively. It is well known that band gaps of WO3 and TiO2 are 2.8 and 3.2 eV, respectively. TiO2 can be excited by photons with wavelength under 387 nm which produces photo-generated electron–hole pairs and show photo catalytic activity in UV light. Degussa P-25 shows higher activity than pure WO3, traditional
Scheme 1. Proposed mechanism of W, N co-doped TiO2 for the degradation of organics in visible light.
N-doped TiO2 and pure TiO2 in UV light irradiation in case of RhB and 2,4-DCP degradation. The doping of N into lattice of TiO2 usually results in the formation of oxygen vacancies in bulk [40,48]. These defects can act as recombination centers for carriers to decrease the photo catalytic efficiency. This is due to the rapid recombination rate of generated electrons and holes introduced by the impurity level. 3.10. Proposed mechanism of degradation of organics by tungsten and nitrogen co-doped TiO2 Scheme 1 gives a mechanism diagram of photo degradation of organics by N, W co-doped TiO2. In co-doped samples, nitrogen as non metal form new states just above the valence band of TiO2 while W species form the energy states just lower the CB of TiO2 simultaneously. These energy states are generated due to the formation of new linkages of N, W and O in our results as N–Ti–O, N–W–O, Ti–O–N and W–O–N (confirmed by XPS study). The band gap is narrowed due to the formation of these new energy states, thus resulting in more visible light excitation of catalysts to produce the photo-induced electrons. Meanwhile, the W (VI) ions in the conduction band can capture the produced electrons to form W (V) and reduce the recombination rate of photo generated electron hole pairs to improve the photo catalytic activity.
Fig. 7. Time dependent photo catalytic degradation of (A) Rhodamine B and (B) 2,4-DCP over different samples (a) pure TiO2, (b) Degussa P-25, (c) pure WO3, (d) 1.5% W, N-TiO2, (e) traditional N-doped TiO2, (f) 6.0% W, N-TiO2, (g) 3.0% W, N-TiO2 and (h) 4.5% W, N-TiO2 for 150 min in visible light.
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4. Conclusions W, N co-doped TiO2 photo catalysts are prepared by simple sol– gel method. 4.5% W, N co-doped TiO2 sample exhibits the best activity as compared to that of Degussa P-25, traditional N-doped TiO2 and pure WO3 in case of visible and UV light. The 4.5% W, N codoped TiO2 shifts light absorption band to the visible range by creating new energy states in the form of N–Ti–O or N–W–O and W–O–N or Ti–O–N linkages. Oxygen vacancies created by codoping process act as trapping agencies for electrons (e) to produce active species. Oxygen vacancies also contribute mainly for the optical absorption. Tungsten and nitrogen co-doping plays the major role to make the composite thermally stable up to 900 8C. Acknowledgements This work has been supported by Higher Education Commission of Pakistan and East China University of Science and Technology, Shanghai, China. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2012. 08.032. References [1] H. Luo, T. Takata, Y. Lee, J. Zhao, K. Domen, Y. Yan, Chem. Mater. 16 (2004) 846– 849. [2] S. Shamaila, A.K.L. Sajjad, F. Chen, J. Zhang, Mater. Res. Bull. 45 (2010) 1372–1382. [3] L. Zang, W. Macyk, C. Lange, W.F. Maier, C. Antonius, D. Meissner, H. Kisch, Chem. Eur. J. 6 (2000) 379–384. [4] S. Shamaila, A.K.L. Sajjad, F. Chen, J. Zhang, Chem. Eur. J. 16 (2010) 13795–13804. [5] W. Zhao, W. Ma, C. Chen, J. Zhao, Z. Shuai, J. Am. Chem. Soc. 126 (2004) 13574– 13575. [6] I. Justicia, P. Ordejon, G. Canto, M.J.L. Ozos, J. Fraxedas, G.A. Battiston, R. Gerbasi, A. Figueras, Adv. Mater. 14 (2002) 1399–1402. [7] A.K.L. Sajjad, S. Shamaila, B.Z. Tian, F. Chen, J.L. Zhang, Appl. Catal. B: Environ. 91 (2009) 397–405. [8] S. Shamaila, A.K.L. Sajjad, F. Chen, J.L. Zhang, Appl. Catal. B: Environ. 94 (2009) 272–280. [9] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [10] S. Sakthivel, H. Kisch, Angew. Chem. Int. Ed. 42 (2003) 4908–4911. [11] T. Ihara, M. Miyoshi, Y. Iriyama, Appl. Catal. B: Environ. 42 (2003) 403–409.
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[12] Y. Sakatani, J. Nunoshige, H. Ando, K. Okusako, H. Koike, T. Takata, J.N. Kondo, M. Hara, K. Domen, Chem. Lett. 32 (2003) 1156–1157. [13] C. Burda, Y. Lou, X. Chen, A.C.S. Samia, J. Stout, J.L. Gole, Nano Lett. 3 (2003) 1049– 1051. [14] A.K.L. Sajjad, S. Shamaila, F. Chen, J.L. Zhang, Chem. Eng. J. 166 (2011) 906–915. [15] A.K.L. Sajjad, S. Shamaila, B.Z. Tian, F. Chen, J.L. Zhang, J. Hazard. Mater. 177 (2010) 781–791. [16] X.Z. Li, F.B. Li, C.L. Yang, W.K. Ge, J. Photochem. Photobiol. A 141 (2001) 209–217. [17] A. Fuerte, M.D. Hernandez-Alonso, A. Iglesias-Juez, A. Martınez-Arias, J.C. Conesa, J. Soria, M. Fernandez-Garcıa, Phys. Chem. Chem. Phys. 5 (2003) 2913–2921. [18] Y. Sakatani, H. Ando, K. Okusako, H. Koike, J. Nunoshige, T. Takata, J.N. Kondo, M. Hara, K. Domen, J. Mater. Res. 19 (2004) 2100–2108. [19] B. Gao, Y. Ma, Y. Cao, W. Yang, J. Yao, J. Phys. Chem. B 110 (2006) 14391–14397. [20] J. Li, J. Xu, W. Dai, H. Li, K. Fan, Appl. Catal. B: Environ. 82 (2008) 233–243. [21] Y. Cong, J.L. Zhang, F. Chen, M. Anpo, D. He, J. Phys. Chem. C 111 (2007) 10618– 10623. [22] H. Hao, J. Zhang, Microporous Mesoporous Mater. 121 (2009) 52–57. [23] C. Shifu, C. Lei, G. Shen, C. Gengyu, Powder Technol. 160 (2005) 198–202. [24] G. Liu, Y.N. Zhao, C.H. Sun, F. Li, H.M. Cheng, G.Q. Lu, Angew. Chem. Int. Ed. 47 (2008) 4516–4520. [25] G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatti, F. Bregani, Langmuir 8 (1992) 1744–1749. [26] L. Lin, R.Y. Zheng, J.L. Xie, Y.X. Zhu, Y.C. Xie, Appl. Catal. B: Environ. 76 (2007) 196– 202. [27] X.Z. Shen, Z.C. Liu, S.M. Xie, J. Guo, J. Hazard. Mater. 162 (2009) 1193–1198. [28] H. Noda, K. Oikawa, T. Ogata, K. Matsuki, H. Kamata, Chem. Soc. Jpn. 8 (1986) 1084–1088. [29] Z.Q. Liu, Y.P. Wou, Z.H. Li, Y.C. Wang, C.C. Ge, Rare Metals 26 (2007) 263–270. [30] R. Urlaub, U. Posset, R.F.T. Thull, J. Non-Cryst. Solids 265 (2000) 276–284. [31] T. Bezrodna, G. Puchkovska, V. Shimanovaska, I. Chashecnnikva, T. Khalyavaka, J. Baran, Appl. Surf. Sci. 214 (2003) 222–231. [32] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, 4th ed., John Wiley and Sons, Inc., New York, 1981, p. 275. [33] L.H.M. Krings, W. Talen, Sol. Energ. Mater. Sol. Cells 54 (1998) 27–37. [34] I.R. Beattie, T.R. Gilson, J. Chem. Soc. A 16 (1969) 2322–2327. [35] M.A. Vuurman, I.E. Wachs, A.M. Hirt, J. Phys. Chem. 95 (1991) 9928–9937. [36] M. Srivasan, T. White, Environ. Sci. Technol. 41 (2007) 4405–4409. [37] S. Sakthivel, M. Janczarek, H. Kisch, J. Phys. Chem. B 108 (2004) 19384–19387. [38] H.X. Li, J.X. Li, Y. Huo, J. Phys. Chem. B 110 (2006) 1559–1565. [39] M. Xing, J. Zhang, F. Chen, Appl. Catal. B 89 (2009) 563–569. [40] J.L. Zhang, Y.M. Wu, M.Y. Xing, S.A.K. Leghari, S. Sajjad, Energy Environ. Sci. 3 (2010) 715–726. [41] J.C.D. Dupin, P.G. Vinatier, A. Levasseur, Phys. Chem. Chem. Phys. 2 (2000) 1319– 1324. [42] I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, K. Takeuch, J. Mol. Catal. A: Chem. 161 (2000) 205–212. [43] E. Serwicka, Colloids Surf. 13 (1985) 287–293. [44] C. Feng, Y. Wang, Z. Jin, J. Zhang, S. Zhang, Z. Wu, Z. Zhang, New J. Chem. 32 (2008) 1038–1046. [45] V.N. Kuznetsov, N. Serpone, J. Phys. Chem. C 113 (2009) 15110–15123. [46] Z.S. Lin, A. Orlov, R.M. Lambert, M.C. Payne, J. Phys. Chem. B 109 (2005) 20948– 20952. [47] V. Puddu, R. Mokaya, G.L. Puma, Chem. Commun. 45 (2007) 4749–4756. [48] Y.M. Wu, H. Liu, J.L. Zhang, F. Chen, J. Phys. Chem. C 113 (2009) 14689–14695.
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Study of new states in visible light active W, N co-doped TiO2 photo catalyst Ahmed Khan Leghari Sajjad, Sajjad Shamaila *, Jinlong Zhang Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 December 2011 Received in revised form 26 July 2012 Accepted 11 August 2012 Available online 18 August 2012
The visible light efficient W, N co-doped TiO2 photo catalysts are prepared by sol–gel method. New linkages of N, W and O are formed as N–Ti–O, N–W–O, Ti–O–N and W–O–N. Electron paramagnetic resonance illustrates the presence of oxygen vacancies in W, N co-doped TiO2 acting as trapping agencies for electrons to produce active species. X-ray photoelectron spectroscopy confirms the presence of new energy states. New linkages and oxygen vacancies are proved to be the main cause for the improved photo catalytic performances. W, N co-doped TiO2 has new energy states which narrow the band gap effectively. W, N co-doped TiO2 is thermally stable and retains its anatase phase up to 900 8C. 4.5% W, N co-doped TiO2 showed superior activity for the degradation of Rhodamine B and 2,4-dichlorophenol as compared to pure titania, Degussa P-25, traditional N-doped TiO2 and pure WO3. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Sol–gel chemistry D. Defects D. Electronic paramagnetic resonance
1. Introduction TiO2 photocatalysis has attracted extensive attention as a promising technique for the degradation of pollutants due to high oxidative power, photo stability and nontoxicity [1]. Generally, TiO2 is only sensitive to UV light due to its large band gap (3.2 eV) [2]. In recent years, great efforts have been made to develop TiO2-based photo catalysts sensitive to visible light in order to make use of solar energy more efficiently in practical applications such as surface modification [3], metal or nonmetal ion doping [4,5], generation of oxygen vacancies [6], combination with other semiconductors [7,8]. Among these strategies, TiO2 doped with nonmetals such as N and C has been considered one of the most promising ways to develop TiO2 photo catalysts sensitive to visible light [9,10]. The visible light sensitive TiO2 photo catalyst is prepared by a hydrolytic process [11]. N-doped TiO2 causes the photo catalytic decomposition of acetaldehyde to CO2 under visible light [12]. Nitrogen doped TiO2 exhibits higher photo catalytic activity for degradation of methylene blue in aqueous solution compared to Degussa P25 under visible light [13]. WO3 coupling has been widely used to improve the photo electrochemical and photo catalytic performance of TiO2 because WO3 is more acidic than TiO2 and can serve as an electron acceptor [14,15]. In case of W (VI), the cation tends to
* Corresponding author. Tel.: +86 92514444024; fax: +86 92514444024. E-mail address: [email protected] (S. Shamaila). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.08.032
migrate toward the surface shifting the isoelectric point to lower values in comparison to pristine TiO2 and reduces the charge recombination [16,17]. Reports on metal and non-metal codoped TiO2 catalysts are seldom. It is suggested that the formation of paramagnetic N species in metal and nitrogen doped samples at interstitial positions in TiO2 lattice was responsible for the visible light response of the catalyst in degradation of acetaldehyde [18]. N-doped TiO2 coupled with tungsten oxide samples were prepared using different methods. However, the preparation methods were too much complicated [19,20]. Previously, iron and nitrogen co-doped TiO2 was synthesized using homogeneous precipitation and then followed with hydrothermal process in the presence of triethylamine [21]. Hao and Zhang prepared iron (III) and nitrogen co-doped mesoporous TiO2 by modified sol–gel method and the nitrogen containing surfactant (dodecylamine) was introduced as a structure directing agent as well as a nitrogen dopant [22]. In this study, W, N co-doped TiO2 photocatalysts are synthesized by simple sol–gel method. New linkages of N–Ti–O, N–W–O, Ti–O–N and W–O–N are responsible for the decrease of band gap energy to enhance its visible light absorption resulting in higher degradation rates. Oxygen vacancies generated during co-doping in W, N codoped TiO2 samples act as trapping sites which effectively decrease recombination rate of the photo induced electrons and holes, thus increase the photo-oxidation efficiency. 4.5% N, W co-doped TiO2 showed far superior activity as compared to P-25, pure TiO2, traditional N-doped TiO2 and pure WO3 for the degradation of RhB and 2,4-DCP in visible and ultraviolet irradiations.
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2. Experimental
2.4. Measurement of photocatalytic activities
2.1. Materials
The photo catalytic performance of W, N-TiO2 nanocomposites was evaluated in the degradation of RhB and 2,4-DCP. The first one is representative of dyes while 2,4-DCP is a highly toxic pollutant. We studied the photo catalytic degradation processes on exposure to visible and UV light. The photo catalytic degradation was carried out in a 100 mL quartz photochemical reactor. The initial concentration of RhB and 2,4-DCP in a quartz reaction vessel was fixed at 20 mgL1 with a catalyst loading of 1.1 g L1. A 1000 W halogen lamp was used as light source and the light from the lamp included beams from ultraviolet and visible light regions. The short-wavelength components (l < 420 nm) of light were cut off using a cut-off glass filter for visible photoreaction. The mercury lamp of 300 W is used for the UV light reaction. During the reaction, a water-cooling system cooled the water-jacketed photochemical reactor to maintain the solution at room temperature. The distance between the lamp and the center of quartz tube was 10 cm. Prior to illumination, reaction mixture was sonicated for 20 min to homogenize and the suspension was magnetically stirred in darkness for 30 min to establish adsorption–desorption equilibrium at room temperature. During irradiation, stirring was maintained to keep the mixture in suspension. At regular intervals, samples were withdrawn and centrifuged to separate photo catalyst for analysis. Then filtered through a 0.22 mm millipore filter to remove the photo catalyst. The photo activities for RhB and 2,4-DCP in dark in the presence of the photo catalyst and under visible light irradiation in the absence of the photo catalyst were also evaluated. The extent of RhB and 2,4-DCP decomposition was determined by measuring the absorbance value at approximately 552 and 283 nm, respectively. The measurements were repeated for the catalyst three times and the experimental error was found to be within 3%.
Tetra butyl titanate (TBT) and acetic acid were used as received. Ammonium tungstate, urea and absolute alcohol were of analytical grade received from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China and used without any further purification. Pure WO3 was supplied from Aldrich. 2.2. Preparation 17 mL TBT was added to the mixture of 120 mL absolute ethanol, 15 mL acetic acid and 5 mL doubly distilled water to form the sol. After aging of sol for 48 h, different additions of ammonium tungstate aqueous solution were dripped to the sol under vigorous stirring according to the required amount of ammonium tungstate until the gel formed. The obtained gel was aged, grinded and treated in vacuum at specific temperature. The samples were calcined at different temperatures. Pure TiO2 was prepared by same procedure without addition of ammonium tungstate. For comparison, nitrogen doped TiO2 was also prepared using urea and designated as traditional N-doped TiO2. 2.3. Catalyst characterization UV–vis diffuse reflectance spectra (DRS) were obtained with a Scan UV-vis-NIR spectrophotometer (Varian Cary 500) equipped with an integrated sphere assembly using BaSO4 as a reflectance sample. X-ray diffraction (XRD) measurements were carried out to investigate the crystallographic properties with a Rigaku D/Max 2550 VB/PC apparatus (Cu Ka1 radiation, l = 1.54056 A˚) at room temperature operated at 40 kV and 100 mA. Diffraction patterns were recorded in the angular range of 20–808. The Scherrer equation was applied to estimate the average crystallite sizes of TiO2 samples:
3. Results and discussion 3.1. Crystal structures
Kl D¼ b cos u where b is the half-height width of the diffraction peak of anatase (101), K = 0.89 is a coefficient, u is the diffraction angle, and l is the X-ray wavelength corresponding to the Cu Ka radiation. D is the average crystallite size of the powder sample in nanometer. Fourier transform infra-red (FT-IR) spectra were recorded by employing a Nicolet 740 FT-IR spectrometer equipped with a TGS detector and a KBr beam splitter. Raman spectra of the sample were recorded by Renishaw inVia Raman spectrometer at room temperature with the excitation wavelength of 514.6 nm. The surface morphologies and elemental mapping were observed by scanning electron microscopy (SEM) and energy dispersive X-ray analyses (EDXA), respectively. The SBET of the samples were determined through nitrogen physical adsorption at 77 K (Micromeritics ASAP 2010). All the samples were degassed at 473 K before the measurement. To investigate the chemical states of the photo catalysts, X-ray photoelectron spectra (XPS) were recorded with PerkinElmer PHI 5000C ESCA system with Al Ka radiation operated at 250 W. The shift of binding energy due to relative surface charging was corrected using the C 1s level at 284.6 eV as an internal standard. Electron paramagnetic resonance (EPR) measurements were recorded at room temperature on a Bruker EMX-8/2.7 with the microwave frequency of 9.85 GHz and power of 6.35 mW. UV–vis absorption spectra of the samples were recorded on Cary 100 UV-vis spectrophotometer.
XRD patterns of pure TiO2 and W, N co-doped TiO2 samples calcined at 500 8C are shown in Fig. 1. Pure TiO2 and W, N co-doped TiO2 samples display crystallized anatase phase which correspond to characteristic diffraction peaks. The intensity of the anatase (1 0 1) peak at 25.38 gradually decreased and its peak width became broader with an increase in dopant. Obviously, the incorporation of dopant into the lattice of TiO2 hindered the crystallization resulting peak width broadening. It is worth noting
Fig. 1. XRD patterns of pure TiO2 and doped samples (a) TiO2, (b) 1.5% W, N-TiO2, (c) 3.0% W, N-TiO2, (d) 4.5% W, N-TiO2 and (e) 6.0% W, N-TiO2.
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that no lower intensity peak of dopants could be observed in addition to the anatase peaks up to 4.5% W, N co-doped TiO2. We can propose that dopant ions may be incorporated into TiO2 lattice which replaced titanium ions or located at interstitial sites. Some of the researcher studied that the concentration of dopants is too low for the XRD to reveal [19,16,23]. In contrast, reflections in addition to the anatase phase are seen in XRD pattern of nanocomposites at higher 6.0% W, N-TiO2. The excess dopant does not enter into the crystal lattices of TiO2 and is uniformly dispersed on TiO2 or covered the surface of TiO2. The particle size of the samples is in the range of 11–17 nm using Scherrer equation. 3.2. Thermal stability The presence of dopant can usually retard the crystal growth and phase transformation from anatase to rutile. Thus doping can enhance the thermal stability of the anatase phase [23]. W–N [21] and B–N [24] co-doped TiO2 have been reported to possess small crystallite sizes and high anatase to rutile phase transformation temperatures with respect to single N-doped TiO2. Fig. S1 shows that 4.5% W, N co-doped TiO2 has larger thermal stability for anatase form up to 900 8C and rutile form appears at 1000 8C. The addition of tungsten increases thermal stability and stabilizes the anatase phase [25] while N has little effect [26]. In fact, rutile form predominates at 800 8C in pure TiO2. The doping quantity of 4.5% W, N could affect the diffraction pattern and changed the particle size. Similarly Ce–N [27] and P–N [26] co-doped TiO2 have been reported to possess small crystallite sizes and high anatase to rutile phase transformation temperatures with respect to solely N doped TiO2. The textural properties are changed with the increase in temperature as shown in Table S1. 3.3. UV–vis diffuse reflectance spectroscopy Fig. 2 shows UV–visible diffuse reflectance spectra of pure TiO2, traditional N-doped TiO2, pure WO3, 3.0 and 4.5% W, N co-doped TiO2. Pure TiO2 exhibits only a strong absorption in ultraviolet region which attributes to the band–band transition (Fig. 2a). Fig. 2b shows diffuse reflectance spectrum of traditional N-doped TiO2 which decreases the band gap resulting into red shift. In Ndoped TiO2, oxygen vacancies are the cause for visible light response [28]. The valence band of N-TiO2 consists of N2p and O2p, that is, only donor energy states exist in the band of N-TiO2 [29]. The absorption onset wavelengths of pure WO3 powder are in the range of 450–480 nm (Fig. 2c). In Fig. 2d, 3.0% W, N co-doped TiO2 shows an additional broad absorption that extends to visible
Fig. 3. (a) EDS of 4.5% W, N-TiO2 and (b) SEM image of 4.5% W, N-TiO2.
region in the range of 600–700 nm. However, 4.5% W, N co-doped TiO2 shows an additional broad absorption that extends throughout the entire visible region (Fig. 2e). Besides the contribution of N species to visible light absorption, W could form dopant energy level within the band gap of TiO2. The concentration of dopants also influences markedly the visible absorption and intensity of the band. The absorption intensity also enhances with the increase of doped W and N concentration. The new linkages N–Ti–O, N–W–O, Ti–O–N and W–O–N and oxygen vacancies formed in W, N-TiO2 act as the additional impurity states or defect energy levels which contribute to the enhanced visible light absorption. 3.4. Energy dispersive X-ray spectra and scanning electron microscopy Energy dispersive X-ray analysis (EDXA) is carried out to determine the presence of elements. The EDX spectrum of W, N codoped TiO2 sample is shown in Fig. 3a. The peaks of W, N, Ti and O can be clearly seen. The presence of signal of Au can be ascribed to Au grid. Fig. 3b shows the SEM image of 4.5% W, N co-doped TiO2. It depicts the loose agglomerates with significant quantity of interparticle voids. 3.5. Fourier transforms infra-red spectra
Fig. 2. DRS of pure TiO2 and different samples (a) pure TiO2, (b) traditional N-doped TiO2, (c) pure WO3, (d) 3.0% W, N-TiO2 and (e) 4.5% W, N-TiO2.
The FT-IR spectra of pure TiO2, 1.5, 3.0 and 4.5% W, N co-doped TiO2 samples are shown in Fig. 4. The intensive and broad band at 550–635 cm1 is ascribed to the stretching vibrations of Ti–O bond [30]. The deformation vibrations at 1600–1630 cm1 (H–O–H) are evidence of large amount of water molecules absorbed on the surface of the catalyst [31]. The presence of large amount of
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Fig. 4. FTIR of pure and different % of doped samples (a) pure TiO2, (b) 1.5% W, NTiO2, (c) 3.0% W, N-TiO2 and (d) 4.5% W, N-TiO2.
hydroxyl groups is beneficial for the photo catalytic process. The band at 2360 cm1 is the characteristic of stretching vibrations of N-bond [32] while the presence of band at 1396 cm1 is an evidence of the absorbed molecular oxygen [30]. The broad bands between 500 and 1000 cm1 are attributed to the framework vibrations of WO3 [33]. The new peak at 1010 cm1 in W, N codoped TiO2 (Fig. 4b) can be attributed to the new linkages of W and N which is absent in pure TiO2 (Fig. 4a). 3.6. Raman spectra The Raman spectra of pure TiO2 and different W, N-TiO2 samples are shown in Fig. S2. The anatase bands observed at 397.0, 516.0 and 638.0 cm1 are assigned to the A1g, B2g and Eg vibrational modes of TiO2, respectively [34]. With an increase in incorporated W and N, the bands from the anatase phase are gradually decreased in intensity and become broader. This is ascribed to the lower crystallinity of anatase phase in consistent with the results of XRD. The peaks at 965–975 cm1 are attributed to the symmetric W5 5O stretching mode of tungsten oxide species linked with nitrogen (Fig. S2b and c). This peak is absent in pure TiO2 as shown in Fig. S2a. These results favorably correspond with the reports [20,35]. 3.7. XPS spectra XPS survey spectra for pure TiO2 and 4.5% W, N co-doped TiO2 are shown in Fig. 5. XPS analysis indicates that W, N co-doped TiO2 samples contain Ti, N, W, O and trace amounts of carbon which originates from the residual carbon in the composite and the adventitious hydrocarbon in the XPS instrument itself. The binding energies (B.E) of Ti 2p3/2 and 2p1/2 of pure titania are located at 458.1 and 463.7 eV, respectively, as shown in Fig. 5a which are attributed to Ti4+ [36]. W, N co-doped TiO2 shows the binding energies (B.E) of Ti 2p3/2 and 2p1/2 located at 458.3 and 464.0 eV. There is slight shifting of binding energy of Ti 2p due to the effect of dopant into the lattice of TiO2. In Fig. 5b, the O 1s band of pure TiO2 shows the binding energy at 529.6 eV while W, N co-doped TiO2 shows the binding energy at 529.8 eV. The peaks at 529.6 and 529.8 eV contain contributions from both the Ti–O and W–O, respectively, having almost similar binding energies. In the present work, energy levels of tungsten are measured in W4d5/2 regions as shown in Fig. 5c. The binding energy of W4d5/2 corresponds to 246.7 eV in our experimental results. WO3 shows typical W4d5/2 and W4d3/2 at 247.8 eV and 260.5 eV [20], respectively. The W4d5/2 peaks of W, N-TiO2 experience the shift to lower binding energy as compared to that of W4d5/2 of pure WO3 which indicates higher
electron density of W atoms in W, N co-doped TiO2 sample. The peaks of N 1s and W 4d shift in opposite directions that further suggest the new linkage of N–W in W, N co-doped TiO2 samples. As shown in Fig. 5d, N 1s peak is broader which ranges from 396 to 404 eV in W, N-TiO2 co-doped samples. According to curve fit data, two peaks are observed at 400.4 eV and 401.6 eV binding energies. The peak at 400.4 eV can be attributed to anionic N– in O– Ti–N or O–W–N linkages [37]. The electro negativity of nitrogen doped into TiO2 lattice is lower than oxygen leading to the reduction of electron density on nitrogen due to presence of more oxidized W species. Therefore, the peak at 400.4 eV in our results is higher than that of Ti–N appearing at 396–399 eV [9,38]. The presumed reason of slight shifting of this peak can be attributed to nitrogen replaces the oxygen in the crystal lattice of TiO2. This also confirms the formation of N–Ti–O bonds or the interstitial nitrogen bound to one lattice oxygen and forms the N–O species. Another peak at 401.6 eV is attributed to the formation of Ti–O–N and W– O–N bonds which further confirmed by curve fit data of O 1s as shown in Fig. 5e. There are two peaks after curve fitting. The peak at 529.8 eV is attributed to O1s in Ti–O or W–O linkages. Other peak at 531.6 eV is attributed to the presence of –O–N and –O–N bonds [20,39,40]. Some researchers suggest that the peak is associated with hydroxyl groups [41]. Consequently, the presence of Ti–O–N suggests that peak at 401.0 eV for N1s region of N-TiO2 should be attributed to the oxidized nitrogen of Ti–O–N or W–O–N. It can be assumed that N1s binding energy may vary from case to case when the TiO2xNx is prepared using different chemical methods [39]. From the above observations, it can be concluded that the states of nitrogen and oxygen in W, N co-doped TiO2 may be various and coexist in the forms of different new linkages as N– W–O, N–Ti–O, Ti–O–N and W–O–N in our case. 3.8. EPR spectra EPR spectra of pure and 4.5% W, N-TiO2 samples are shown in Fig. 6 recorded at 298 K. Pure TiO2 shows no significant signal (Fig. 6a). 4.5% W, N co-doped TiO2 sample (Fig. 6b) shows a new significant signal due to shallow donor state at g = 2.002. This result suggests that co-doping favors the formation of oxygen vacancies. Nakamura et al. [42] reported that the symmetrical and sharp EPR signal at g = 2.004 detected on plasma treated TiO2 arose from the electron trapped on the oxygen vacancy. Serwicka et al. [43] observed a sharp signal at g = 2.003 on the vacuum reduced TiO2 at 673–773 K. They attributed this signal to a bulk defect, probably an electron trapped on an oxygen vacancy. Interestingly, similar signal (g = 2.004) was also found in N-doped TiO2 by Feng et al. [44]. Serpone and co-workers assigned the signal at g = 2.003–2.005 to the one electron trapped on the oxygen vacancy or referred to as an F center vacancy [45]. It is reported that F center vacancy located below the conduction band edge of TiO2 results in reduced TiOx and anion doped TiO2 photo catalyst are responsive to visible light [45]. Oxygen vacancies contribute mainly for the optical absorption in this region [46]. There is only one signal with g = 2.002 in 4.5% W, N co-doped TiO2 sample which attributes to the electron trapped on the oxygen vacancy (F centers). It is reasonable to assume that F center vacancy actually exists in the W, N modified TiO2 and existence of oxygen vacancy results in the sensitivity of photo catalyst to visible light. In Fig. 6c, the intensity of EPR signal peak increases obviously after visible light illumination which indicates the existence of free oxygen vacancy due to its trapping of photo-excited electron. 3.9. Photo catalytic degradation of Rhodamine B and 2,4-DCP The photo degradation of RhB and 2,4-DCP is employed to evaluate the photo catalytic activities under visible and UV
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Fig. 5. XPS spectra of pure TiO2 and 4.5% W, N-TiO2 samples (a) Ti 2p, (b) O 1s, (c) W4d, (d) curve fit of N 1s 4.5% W, N-TiO2, (e) curve fit of O 1s of 4.5% W, N-TiO2 and (f) full region XPS survey of 4.5% W, N co-doped TiO2.
irradiations. 4.5% W, N-TiO2 shows 100% photo degradation of RhB and 70.0% 2,4-DCP in visible light in 150 min. W, N co-doped TiO2 samples are more photoactive than bare TiO2, Degussa P-25, traditional N-doped titania and pure WO3 as shown in Fig. 7. The expanded photo activity under visible light is due to W and N codoping and oxygen vacancies. The N–Ti–O or N–W–O linkages forms new energy bands which induce the narrowing of band gap of TiO2. The metal and non metal doped TiO2 decrease the recombination rate of excited electrons and holes. It is proposed that oxygen vacancies are the major cause of visible light response.
The doped W ions with high positive charge act as an accepter and favors to separate photo excited electron/hole pairs [47]. W (VI) species in the present W, N-TiO2 catalysts are also considered to act as trapping site by accepting the photo excited electrons from the TiO2 valence band and then generating W (V). Nitrogen creates the energy levels above the valence band of TiO2. In brief, these two processes co-operated to improve the efficiency of absorbing visible light and form photo induced electrons and holes which both enhance the W, N-TiO2 photo catalytic activity under visible light. For WO3, theoretically, it can be excited by photons with
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Fig. 6. EPR spectra of different samples (a) pure TiO2, (b) 4.5% W, N co-doped TiO2 and (c) 4.5% W, N co-doped TiO2 after visible light illumination of 15 min.
wavelength under 443 nm. But it shows only low photo catalytic activity under visible light because of high charge recombination. Traditional N-doped TiO2 shows higher photo activity as compared to pure TiO2, pure WO3 and Degussa P-25 due to the extension of visible light response to some extent. N-doped TiO2 is responsible for the decrease of band gap [19]. In W, N co-doped TiO2, visible light response becomes stronger with increase of tungsten doping and the generation of linkage of N–W–O and N–Ti–O. It is well known that WO3 retains much higher Lewis surface acidity than TiO2 and has higher affinity for chemical species having unpaired electrons. Therefore, it is much easier for nitrogen species to coordinate with W dispersed on the surface than TiO2 surface itself to form N–W–O linkages as observed in our XPS data which also contributes to visible light responses. When the amount of dopant reaches to 6.0%, the photo catalytic activity of the W, N co-doped TiO2 decrease even though the visible light absorption still increase. It may be due to the charge transfer of the composite catalysts to O2 or target molecules could be retarded with too much dopant. Fig. S3(a and b) shows the degradation of RhB and 2,4-DCP in UV light, respectively. It is well known that band gaps of WO3 and TiO2 are 2.8 and 3.2 eV, respectively. TiO2 can be excited by photons with wavelength under 387 nm which produces photo-generated electron–hole pairs and show photo catalytic activity in UV light. Degussa P-25 shows higher activity than pure WO3, traditional
Scheme 1. Proposed mechanism of W, N co-doped TiO2 for the degradation of organics in visible light.
N-doped TiO2 and pure TiO2 in UV light irradiation in case of RhB and 2,4-DCP degradation. The doping of N into lattice of TiO2 usually results in the formation of oxygen vacancies in bulk [40,48]. These defects can act as recombination centers for carriers to decrease the photo catalytic efficiency. This is due to the rapid recombination rate of generated electrons and holes introduced by the impurity level. 3.10. Proposed mechanism of degradation of organics by tungsten and nitrogen co-doped TiO2 Scheme 1 gives a mechanism diagram of photo degradation of organics by N, W co-doped TiO2. In co-doped samples, nitrogen as non metal form new states just above the valence band of TiO2 while W species form the energy states just lower the CB of TiO2 simultaneously. These energy states are generated due to the formation of new linkages of N, W and O in our results as N–Ti–O, N–W–O, Ti–O–N and W–O–N (confirmed by XPS study). The band gap is narrowed due to the formation of these new energy states, thus resulting in more visible light excitation of catalysts to produce the photo-induced electrons. Meanwhile, the W (VI) ions in the conduction band can capture the produced electrons to form W (V) and reduce the recombination rate of photo generated electron hole pairs to improve the photo catalytic activity.
Fig. 7. Time dependent photo catalytic degradation of (A) Rhodamine B and (B) 2,4-DCP over different samples (a) pure TiO2, (b) Degussa P-25, (c) pure WO3, (d) 1.5% W, N-TiO2, (e) traditional N-doped TiO2, (f) 6.0% W, N-TiO2, (g) 3.0% W, N-TiO2 and (h) 4.5% W, N-TiO2 for 150 min in visible light.
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4. Conclusions W, N co-doped TiO2 photo catalysts are prepared by simple sol– gel method. 4.5% W, N co-doped TiO2 sample exhibits the best activity as compared to that of Degussa P-25, traditional N-doped TiO2 and pure WO3 in case of visible and UV light. The 4.5% W, N codoped TiO2 shifts light absorption band to the visible range by creating new energy states in the form of N–Ti–O or N–W–O and W–O–N or Ti–O–N linkages. Oxygen vacancies created by codoping process act as trapping agencies for electrons (e) to produce active species. Oxygen vacancies also contribute mainly for the optical absorption. Tungsten and nitrogen co-doping plays the major role to make the composite thermally stable up to 900 8C. Acknowledgements This work has been supported by Higher Education Commission of Pakistan and East China University of Science and Technology, Shanghai, China. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2012. 08.032. References [1] H. Luo, T. Takata, Y. Lee, J. Zhao, K. Domen, Y. Yan, Chem. Mater. 16 (2004) 846– 849. [2] S. Shamaila, A.K.L. Sajjad, F. Chen, J. Zhang, Mater. Res. Bull. 45 (2010) 1372–1382. [3] L. Zang, W. Macyk, C. Lange, W.F. Maier, C. Antonius, D. Meissner, H. Kisch, Chem. Eur. J. 6 (2000) 379–384. [4] S. Shamaila, A.K.L. Sajjad, F. Chen, J. Zhang, Chem. Eur. J. 16 (2010) 13795–13804. [5] W. Zhao, W. Ma, C. Chen, J. Zhao, Z. Shuai, J. Am. Chem. Soc. 126 (2004) 13574– 13575. [6] I. Justicia, P. Ordejon, G. Canto, M.J.L. Ozos, J. Fraxedas, G.A. Battiston, R. Gerbasi, A. Figueras, Adv. Mater. 14 (2002) 1399–1402. [7] A.K.L. Sajjad, S. Shamaila, B.Z. Tian, F. Chen, J.L. Zhang, Appl. Catal. B: Environ. 91 (2009) 397–405. [8] S. Shamaila, A.K.L. Sajjad, F. Chen, J.L. Zhang, Appl. Catal. B: Environ. 94 (2009) 272–280. [9] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [10] S. Sakthivel, H. Kisch, Angew. Chem. Int. Ed. 42 (2003) 4908–4911. [11] T. Ihara, M. Miyoshi, Y. Iriyama, Appl. Catal. B: Environ. 42 (2003) 403–409.
3089
[12] Y. Sakatani, J. Nunoshige, H. Ando, K. Okusako, H. Koike, T. Takata, J.N. Kondo, M. Hara, K. Domen, Chem. Lett. 32 (2003) 1156–1157. [13] C. Burda, Y. Lou, X. Chen, A.C.S. Samia, J. Stout, J.L. Gole, Nano Lett. 3 (2003) 1049– 1051. [14] A.K.L. Sajjad, S. Shamaila, F. Chen, J.L. Zhang, Chem. Eng. J. 166 (2011) 906–915. [15] A.K.L. Sajjad, S. Shamaila, B.Z. Tian, F. Chen, J.L. Zhang, J. Hazard. Mater. 177 (2010) 781–791. [16] X.Z. Li, F.B. Li, C.L. Yang, W.K. Ge, J. Photochem. Photobiol. A 141 (2001) 209–217. [17] A. Fuerte, M.D. Hernandez-Alonso, A. Iglesias-Juez, A. Martınez-Arias, J.C. Conesa, J. Soria, M. Fernandez-Garcıa, Phys. Chem. Chem. Phys. 5 (2003) 2913–2921. [18] Y. Sakatani, H. Ando, K. Okusako, H. Koike, J. Nunoshige, T. Takata, J.N. Kondo, M. Hara, K. Domen, J. Mater. Res. 19 (2004) 2100–2108. [19] B. Gao, Y. Ma, Y. Cao, W. Yang, J. Yao, J. Phys. Chem. B 110 (2006) 14391–14397. [20] J. Li, J. Xu, W. Dai, H. Li, K. Fan, Appl. Catal. B: Environ. 82 (2008) 233–243. [21] Y. Cong, J.L. Zhang, F. Chen, M. Anpo, D. He, J. Phys. Chem. C 111 (2007) 10618– 10623. [22] H. Hao, J. Zhang, Microporous Mesoporous Mater. 121 (2009) 52–57. [23] C. Shifu, C. Lei, G. Shen, C. Gengyu, Powder Technol. 160 (2005) 198–202. [24] G. Liu, Y.N. Zhao, C.H. Sun, F. Li, H.M. Cheng, G.Q. Lu, Angew. Chem. Int. Ed. 47 (2008) 4516–4520. [25] G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatti, F. Bregani, Langmuir 8 (1992) 1744–1749. [26] L. Lin, R.Y. Zheng, J.L. Xie, Y.X. Zhu, Y.C. Xie, Appl. Catal. B: Environ. 76 (2007) 196– 202. [27] X.Z. Shen, Z.C. Liu, S.M. Xie, J. Guo, J. Hazard. Mater. 162 (2009) 1193–1198. [28] H. Noda, K. Oikawa, T. Ogata, K. Matsuki, H. Kamata, Chem. Soc. Jpn. 8 (1986) 1084–1088. [29] Z.Q. Liu, Y.P. Wou, Z.H. Li, Y.C. Wang, C.C. Ge, Rare Metals 26 (2007) 263–270. [30] R. Urlaub, U. Posset, R.F.T. Thull, J. Non-Cryst. Solids 265 (2000) 276–284. [31] T. Bezrodna, G. Puchkovska, V. Shimanovaska, I. Chashecnnikva, T. Khalyavaka, J. Baran, Appl. Surf. Sci. 214 (2003) 222–231. [32] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, 4th ed., John Wiley and Sons, Inc., New York, 1981, p. 275. [33] L.H.M. Krings, W. Talen, Sol. Energ. Mater. Sol. Cells 54 (1998) 27–37. [34] I.R. Beattie, T.R. Gilson, J. Chem. Soc. A 16 (1969) 2322–2327. [35] M.A. Vuurman, I.E. Wachs, A.M. Hirt, J. Phys. Chem. 95 (1991) 9928–9937. [36] M. Srivasan, T. White, Environ. Sci. Technol. 41 (2007) 4405–4409. [37] S. Sakthivel, M. Janczarek, H. Kisch, J. Phys. Chem. B 108 (2004) 19384–19387. [38] H.X. Li, J.X. Li, Y. Huo, J. Phys. Chem. B 110 (2006) 1559–1565. [39] M. Xing, J. Zhang, F. Chen, Appl. Catal. B 89 (2009) 563–569. [40] J.L. Zhang, Y.M. Wu, M.Y. Xing, S.A.K. Leghari, S. Sajjad, Energy Environ. Sci. 3 (2010) 715–726. [41] J.C.D. Dupin, P.G. Vinatier, A. Levasseur, Phys. Chem. Chem. Phys. 2 (2000) 1319– 1324. [42] I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, K. Takeuch, J. Mol. Catal. A: Chem. 161 (2000) 205–212. [43] E. Serwicka, Colloids Surf. 13 (1985) 287–293. [44] C. Feng, Y. Wang, Z. Jin, J. Zhang, S. Zhang, Z. Wu, Z. Zhang, New J. Chem. 32 (2008) 1038–1046. [45] V.N. Kuznetsov, N. Serpone, J. Phys. Chem. C 113 (2009) 15110–15123. [46] Z.S. Lin, A. Orlov, R.M. Lambert, M.C. Payne, J. Phys. Chem. B 109 (2005) 20948– 20952. [47] V. Puddu, R. Mokaya, G.L. Puma, Chem. Commun. 45 (2007) 4749–4756. [48] Y.M. Wu, H. Liu, J.L. Zhang, F. Chen, J. Phys. Chem. C 113 (2009) 14689–14695.