A novel TiO2 with a large amount of bulk intrinsic defects—Visible-light-responded photocatalytic activity induced by foreign trap

Science Bulletin, Mar 2013

A novel TiO2 (anatase) containing a large amount of single electron trapped oxygen vacancies (SETOV) was prepared by dehydration of titanic acid nanotubes. This novel TiO2 contains high concentration intrinsic defects in bulk structure, while its surface still remains the stoichiometric structure to protect them. And this novel TiO2 itself has the visible light absorption without any doping, so we call it as the third generation of TiO2. However, it is regretted that this novel TiO2 (A) only has photocatalytic activity under UV light irradiation, and was inactive for the visible light. The true reasons for this phenomenon were investigated by the transient IR absorption and photoluminescence spectra. Through constructing the foreign electron traps (PdO, PtO2), the photocatalytic oxidation of propylene under visible light irradiation was successfully achieved. The removal yield of propylene (C3H6) reached 7.6% and 28% on 2 wt.% PtO2/novel TiO2 and 2 wt.% PdO/novel TiO2, respectively. By comparison with the noble metal electron traps (Pt, Pd), we found that the effective foreign electron traps need to satisfy two conditions: (1) its work function should situate in the range of Eg(TiO2); (2) O2 adsorbes on it undissociatively. This work opens up a new route for the investigation of solar-energy-available TiO2.

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A novel TiO2 with a large amount of bulk intrinsic defects—Visible-light-responded photocatalytic activity induced by foreign trap

LI QiuYe 0 ZHANG JiWei 0 JIN ZhenSheng 0 FENG CaiXia 0 ZHANG JingWei 0 WU ZhiShen 0 ZHANG ZhiJun 0 0 Key Laboratory for Special Functional Materials, Henan University , Kaifeng 475004, China A novel TiO2 (anatase) containing a large amount of single electron trapped oxygen vacancies (SETOV) was prepared by dehydration of titanic acid nanotubes. This novel TiO2 contains high concentration intrinsic defects in bulk structure, while its surface still remains the stoichiometric structure to protect them. And this novel TiO2 itself has the visible light absorption without any doping, so we call it as the third generation of TiO2. However, it is regretted that this novel TiO2 (A) only has photocatalytic activity under UV light irradiation, and was inactive for the visible light. The true reasons for this phenomenon were investigated by the transient IR absorption and photoluminescence spectra. Through constructing the foreign electron traps (PdO, PtO2), the photocatalytic oxidation of propylene under visible light irradiation was successfully achieved. The removal yield of propylene (C3H6) reached 7.6% and 28% on 2 wt.% PtO2/novel TiO2 and 2 wt.% PdO/novel TiO2, respectively. By comparison with the noble metal electron traps (Pt, Pd), we found that the effective foreign electron traps need to satisfy two conditions: (1) its work function should situate in the range of Eg(TiO2); (2) O2 adsorbes on it undissociatively. This work opens up a new route for the investigation of solar-energy-available TiO2. - TiO2 became the most popular photocatalyst since the discovery of the Fujishima-Honda effect [1]; however, it can only work under UV light. For utilizing more visible light of the solar spectrum, a great deal of work on doping of TiO2 has been done by many scientists. Doping generated a large amount of non-intrinsic defects within TiO2 crystal lattice, people called this doped TiO2 as the second generation of TiO2 [2]. Two kinds of materials are often doped in TiO2 to extend the optical absorbance: (1) metal ions, such as Cr, V, Fe, Pb, Cu, etc. [36]. They can extend the photoresponse of TiO2 to the visible light region, however, the doped metal ions can also play the role of the recombination centers to reduce its photocatalytic activity [7]; (2) nonmetalic ions, such as C, N, F, P, S, etc. Recently, much attention has been focused on the nitrogen doping [813]. It should be emphasized is that the origin of the visible-lightresponded photocatalytic activity of N-doped TiO2 still remains disputable [1419]. The redox potential of the doped nitrogen (e.g. NO) exists within Vcb to Vvb of TiO2 [20], so the photoelectrochemical stability of the N-doped TiO2 may be a question for the practical application. In addition to the non-intrinsic defects can make the photo-response of TiO2 red-shift, the intrinsic defects can also play the same role. As well known, when TiO2 was reduced in H2 atmosphere or treated under vacuum at high temperature, two kinds of intrinsic defects (i.e. oxygen vacancies denoted as Vo, and Ti3+) would form on TiO2 surface simultaneously [2123]. The energy levels of both in The Author(s) 2013. This article is published with open access at Springerlink.com trinsic defects are within Eg (TiO2). Vo and Ti3+ were easily to react with the ambient oxygen to recover the original surface stoichiometric state of TiO2, so they are not stable and cannot be utilized in the practical visible photocatalysis. Then, we wonder how to prepare a TiO2 with intrinsic defects in bulk and stoichiometric structure on surface. To the best of our knowledge, there have no report about it up to now. In 1998, Kasuga et al. [24] obtained a nanotube material by treating anatase TiO2 powders with a 10 mol/L NaOH aqueous solution at 110C. Following his prepared method, we confirmed the detailed chemical structure of this nanotube material as Na2Ti2O2(OH)2. Nanotube Na2Ti2O2(OH)2 can converte to nanotube titanic acid (H2Ti2O2(OH)2, NTA) in a pH 1 HCl solution, and both of them belong to the orthorhombic crystalline system [2527]. In our former work [2831], the formation mechanism, optical property and band structure of the nanotube Na2Ti2O2(OH)2 or H2Ti2O2(OH)2 have been investigated systematically and intensively. When NTA was dehydrated at an elevated temperature in air, a novel TiO2 (anatase) was obtained. ESR determination revealed that it contains a large amount of the intrinsic defects: single electron trapped oxygen vacancy (SETOV, denoted as Vo) and Ti3+ [2831]. The concentration of Vo obtained from the hydration of NTA at 400C reached 4.61024 spin/m3 [32]. Through investigating the relationship between the Stokes shift and the excitation wavelength of the visible light, we found that SETOV with high concentration forms a sub-band within Eg (TiO2). The top and bottom of the sub-band is 2.27 and 1.79 eV higher than the valence band of TiO2, and the gap of the sub-band is 0.48 eV [33]. This kind of novel TiO2 (A) possesses not only a broad optical response from UV to visible light region, but also a very good chemical stability in air. We call it as the third generation of TiO2. This kind of TiO2 is very different with the conventional TiO2 (first generation) or doped TiO2 (second generation). The bulk of the third generation of TiO2 has a large amount of the intrinsic defects, and the surface remains the stoichiometric structure to protect them [34]. However, it is regretted that this novel TiO2 (A) only has UV photocatalytic activity, and was inactive for the visible light. In this paper, we studied the structure, optical properties and lifetime of the conduction band electrons of the novel TiO2 (A), and found out the true reason of its inactive photocatalytic activity in visible light. Based on the above study, we adopted some foreign traps on the novel TiO2 (A) to induce the transfer of the bulk visible-light-generated electrons to its surface, and obtained a visible-light-responded photocatalyst successfully. 1.1 Preparation of the novel TiO2 (A) and PtO2/PdO loaded novel TiO2 (A) Nanotube titanic acid (H2Ti2O4(OH)2, NTA) was prepared using concentrated NaOH and TiO2 as the raw materials, according to a typical procedure described previously [25,26]. The novel TiO2 (A) was obtained by treating NTA in air at T400C for 23 h. The PtO2/PdO loaded novel TiO2 (A) was prepared by simple grinding the mixture of PtO2 (or PdO) and the novel TiO2 (A). PdO (99.9%) was purchased from Alfa Aesar company. And PtO2 was presented by Lanzhou Institute of Chemical Physics, CAS. Pd0, Pt0 were the reduced products of PdO, PtO2 in H2 flow at 400C for 4 h. The loaded amount of the cocatalyst was 2 wt.%. Diffuse reflectance spectra (DRS) were recorded on a Shimadzu U-3010 spectrometer. X-Ray diffraction (XRD) patterns were measured on a Philips XPert Pro X-ray diffractometer (Holland) (Cu K radiation, 2 range 590, scan step size 0.08, time per step 1.0 s, generator voltage 40 kV, tube current 40 mA). Photoluminescence (PL) spectra were determined using a SPEX F 212 spectrometer. In-situ IR spectra were recorded on a Nicolet 870 FTIR spectrometer with a MCT, the IR cell with the sample was evacuated and dry air was then introduced. X-ray photoelectron spectroscopy (XPS) characterizations were performed using a Shimadzu Axis Ultra multifunctional X-ray photoelectron spectrometer. The energy scale of the spectra was corrected using the binding energy of adventitious carbon C1s=284.8 eV; quantitative analysis of the surface elemental composition was accomplished by a computer program using the XPS sensitivity factors provided by the Shimadzu Co. Evaluation of photocatalytic activity The photocatalytic activity of catalysts was evaluated by monitoring the photocatalytic oxidation reaction of propylene. The 301 mg of photocatalyst was dispersed in 10 mL of de-ionized water and spread on the rough surface of a glass plate (0.9 cm11 cm0.2 cm). The glass plate was then put into a flat quartz tube reactor with a dead space of 14 mL. A 500 W xenon lamp was used as the visible light source. Between the xenon lamp and reactor were inserted a cutoff filter (420 nm) and a water cell to eliminate the ultraviolet and infrared light, outputting visible light (420 nm) with an intensity of about 8.0 mW/cm2. A black-light lamp (4 W, main wavelength of 365 nm) was used as the ultraviolet source, and its light intensity was determined to be 2.5 mW/cm2. The feed gas was made up of pure C3H6 and dry air, in which the ratio of C3H6 and air was 600/106 (that is, 600 ppm), and was stored in a high-pressure cylinder. The concentration of C3H6 and CO2 was determined by a chromatographic method (on a Shimadzu GAS CHROMATOGRAPH GC-9A, which was equipped with a GDX-502 column, a Flame ionization detector (FID) and a reactor loaded with Ni catalyst for the methanization of CO2, for in-situ analysis). The sensitivity for C3H6 analysis was 1 ppm. The C3H6 removal (x)=(C0C)/C0100%, where C0 refers to the initial concentration of C3H6. Results and discussion Transient IR absorption The third generation of TiO2 derived from dehydrate NTA at different elevated temperature (400CT700C) showed both UV light and visible light response (Figure 1(a)), and they belonged to the anatase phase structure (Figure 1(b)). However, this kind of novel TiO2 (A) is only UV active in photocatalytic oxidation of C3H6, and it did not show any activity under visible light (Figure 2). This novel TiO2 (A) has visible light absorption, but why it has no photocatalytic activity? At one time, we thought that if this phenomenon was possibly resulted from its strong photo-luminescence (PL) under visible light. However, this doubt was denied after the transient IR absorption spectra of both P25-TiO2 and novel TiO2 (A) were determined. The transient IR absorption of 10003000 cm1 is a powerful tool for investigating the lifetime of the photo-generated electrons in the conduction band [3537]. Figure 3 illustrated the transient IR absorption curves of P25 and novel TiO2 (A) determined at the identical conditions. Using ns pulse of 355 nm laser as the excitation light, the electron concentrations (proportional to absorbance A) obtained in both conduction bands are approximately same, but the decay rates are very different. The time decayed to zero is ca. 0.2 s for P25-TiO2, while it is ca. 8.5 s for the novel TiO2 (A). It indicates that the lifetime of conduction band electrons () of the latter is 43 times longer than that of the former. There are two routes for the decay of photo-generated electrons in conduction band: (1) bulk recombination, (2) transfer to the surface to conduct the chemical reaction. If there were no reactants on the surface, would be controlled by the bulk recombination rate (rrecomb, 1/rrecomb). Bulk recombination can be radiative to emit fluorescence or/and non-radiative to release heat, which only depends on the semiconductor structure and is irrelevant to the activity of photocatalysts. The photocatalytic oxidation results of propylene (Figure 2) illuminated that when the reactants present, though of P25TiO2 is very shorter, its UV light-generated conduction band electrons can transfer to surface to conduct reactions, but for longer of visible light-generated conduction band electrons of novel TiO2 (A) cant, what is the reason? Surface reconstruction and electron trap In the gas-solid photocatalytic reaction of TiO2, the oxidized removal of organic pollutants was realized by using superoxide O2(a) as the oxidant [37,36]. The formation of O2(a) was the preliminary step, then through a complex intermeFigure 1 The UV-vis DRS spectra (a) and XRD spectra (b) of the titanic acid nanotubes and the novel TiO2. Figure 2 The photocatalytic activity of P25-TiO2 and novel TiO2 under UV and visible light irradiation. RH O2 (a)+h+ CO2 +H2O Reaction 1 only takes place when the bulk photo-generated electron was transferred to the surface. The precondition for Figure 3 Transient IR absorption spectra of P25 and the novel TiO2. transfer the bulk electrons out is that there must have electron traps on TiO2 surface, whose energy level should be lower than the edge of the conduction band of TiO2. The stoichiometric TiO2 surface consists of Ti4+and O2 ions. The energy difference between Ti3d LUMO orbital and O2p HOMO orbital equals to 3.2 eV. When the energy of the incident light is larger than 3.2 eV, O2p electron would be excited to Ti3d orbital to form Ti3+ and O ions. According to the Frank-Condons principle, the change of valence state of surface ions will result in the surface reconstruction to generate new sites with lower energy than that of the conduction band edge [38]. The new sites can play the role of electron traps to accept the photo-generated electrons coming from the conduction band. Figure 4(a) and (b) indicated that a PL peak (PL=470 nm corresponding to EPL=2.6 eV) appeared when the excitation wavelength (ex) was from 360 to 390 nm for both P25-TiO2 and novel TiO2 (A). This PL peak belongs to the emission of esurf h+vb recombination, which can only happen under UV irradiation. Its energy potential is lower than that of the conduction band edge of TiO2. By means of the second harmonic generation, Shultz et al. [39] also evidenced that Ti3+ formed on TiO2 surface under UV irradiation. Therefore it can be concluded that in the course of gas-solid photocatalysis, the UV light irradiated on any one TiO2 brings out two effects: (1) to excite the valence band electron to conduction band; (2) to provide electron traps on the surface. While the visible light irradiated on novel TiO2 (A) only gets effect (1): the valence band electron will jump to the conduction band using the sub-band formed by the single-electron-trapedoxygen-vacancy as the springboard, and the surface reconstruction for producing electron traps does not occur (Figure 4(c)). So, novel TiO2 (A) showed the visible light response, but no visible-light-responded photocatalytic activity. Constructing the foreign electron trap Under visible light irradiation, the surface of the novel TiO2 (A) itself cannot reconstruct to form the electron traps. It enlightens us to adopt foreign electron trap to solve the question of transferring the electrons from the conduction band to the surface to conduct the catalytic reaction. The thermodynamic requisite for foreign trap materials is that their work function must be situated in the range of Eg(TiO2). The noble metals (Pt, Pd) and metal oxides (PtO2, PdO) are often used as the electron traps to improve the separation of the charge carriers. All of their work functions are situated lower than the conduction band of TiO2 [20,40,41]. Figure 5 shows their XPS spectra. As well known, Pt0 and Pd0 are often used as the co-catalyst of TiO2 to improve the activity in many photocatalytic reactions. This was due to that Pt0 and Pd0 can act as the effective electron traps to increase the photo-excited charge carrier separation of TiO2. According to this consideration, we think that Pt0/novel TiO2 (A) and Pd0/novel TiO2 (A) should be active under visible light irradiation. However, the actual results were unexpected. As shown in Figure 6, both Pt0/novel TiO2 (A) and Pd0/novel TiO2 (A) did not show any photocatalytic activity for C3H6 oxidation. Selection of the photocatalytic probe reaction should consider the real reaction mechanism. As well known, the dissociative adsorption of O2 easily takes place on the noble metal (Pt0, Pd0), even at very low temperature (eq. (3)) [42,43]. Figure 4 (a) PL spectra of P25-TiO2 (ex=360650 nm); (b) Deconvoluted PL spectra (ex=360390 nm)2 of novel TiO2 (A); (c) PL spectra of novel TiO2 (A) (ex=360650 nm). The XPS spectra of foreign electron traps of PtO2 (a), Pt0 (b), PdO (c), Pd0 (d). If photo-generated electrons transferred to Pt0/Pd0 surface, it would be beneficial to O2 dissociation (OO bond rupture energy: in O2, 5.22 eV; in O2, 4.1 eV) [44,45]. O2 Pd0 , Pt0 2Oa O2 Pd0ec,bPt0 O2 Oa Oa O2(a) cannot form on Pt0, Pd0 (eq. (4))therefore the C3H6 oxidation removal (eq. (2)) certainly does not happen. However, the dissociated adsorption of O2 will not take place on PtO2, and PdO [41,46]. To replace Pt0, Pd0 with PtO2, PdO, as expected, the visible photocatalytic activities on both 2 wt.% PtO2/novel TiO2 (A) and 2 wt.% PdO/novel TiO2 (A) were obtained (Figure 6). The removal yield of propylene (C3H6) reached 7.6% and 28%, respectively. The effect of the flow rate of the feed gas on the removal yield of C3H6 and CO2 formation rate are also investigated (Figure 7), where the specific rate constant k=3.3 h1. As a comparison experiment, the photocatalytic reaction of propylene on 2% PdO-P25 was conducted. As shown in Figure 6, no photocatalytic activity was found on 2% PdO-P25. These results indicated that the visible-light-excited electrons come from the novel TiO2, and the PdO and PtO2 were only acted as the electron traps. For further improve the photocatalytic activity and push the practical application, preparing the nanostructured PtO2/ PdO loaded novel TiO2 (A) and attempting other cheap and efficient foreign-trap material is under investigation. In summary, we obtained the following conclusions: (1) A Figure 7 Visible-light photocatalytic activity of 2 wt.% PdO/Novel TiO2. (a) Dependence of C3H6 conversion on feed gas rate; (b) dependence of CO2 formation on feed gas flow rate; (c) the specific rate constants. novel TiO2 (A) derived from the dehydrate NTA showed the visible light response, but no photocatalytic activity for C3H6 oxidation. The reason was possibly that the sub-band of the novel TiO2 (A) plays the role of mediator for the transition of valence band electron to the conduction band under visible light irradiation, while the surface reconstruction for producing electron traps does not happen. The bulk photogenerated electrons cannot be transferred to the surface, so the photocatalytic reaction cannot proceed. (2) The visible photocatalytic oxidation of C3H6 was successively realized by adding foreign electron traps (PtO2, PdO) on the novel TiO2 (A). The removal yield of C3H6 on PtO2/novel TiO2 (A) and PdO/novel TiO2 (A) reached 7.6% and 28%, respectively. At the same time, a number of mineralization products (CO2) were obtained. The prerequisite for the effective foreign electron traps need to satisfy two conditions: its work function situated in the range of Eg(TiO2) and O2 adsorbed on it undissociatively. (3) The finding of the reason that the novel TiO2 (A) having the visible light response but no photocatalytic activity opens up a new route for the investigation of solar-energy-available TiO2. The novel TiO2 (A) with high concentration of intrinsic defects in bulk and stoichiometry on surface was called the third generation of TiO2. This work was supported by the National Natural Science Foundation of China (21103042), the Specialized Research Fund for the Doctoral Program of Higher Education (20114103120001) and the Scientific Research Foundation of Henan University (2010YBZR013). The authors are indebted to Porf. JianJun Yang and Mr. GuanJun Cuis help for the measurements of transient IR absorption spectra.


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QiuYe Li, JiWei Zhang, ZhenSheng Jin, CaiXia Feng, JingWei Zhang, ZhiShen Wu, ZhiJun Zhang. A novel TiO2 with a large amount of bulk intrinsic defects—Visible-light-responded photocatalytic activity induced by foreign trap, Science Bulletin, 2013, 1675-1681, DOI: 10.1007/s11434-013-5682-9