N Codoped Anatase TiO2 - The Journal

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A Novel Method to Prepare B/N co-doped Anatase TiO Jingzhong Zhao, Lei Zhang, Weiqiang Xing, and Kathy Lu

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512837n • Publication Date (Web): 19 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

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The Journal of Physical Chemistry

A Novel Method to Prepare B/N Co-doped Anatase TiO2 Jingzhong Zhaoa*; Lei Zhanga; Weiqiang Xinga; Kathy Lub (a1School of Materials and Engineering, Xi′an University of Technology, Xi′an 710048; China) ( bDepartment of Materials Science and Engineering, Virginia Tech, Blacksburg, Virginia 24061, USA) E-mail: Jingzhong Zhao: [email protected] Lei Zhang: [email protected] Weiqiang Xing: [email protected] Kathy Lu: [email protected]

*Corresponding author. Telephone: +86 29 13909218397 E-mail address: [email protected] 1

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A Novel Method to Prepare B/N Co-doped Anatase TiO2 Abstract：B/N co-doping is an effective way to narrow the band gap of TiO2 to enhance the photocatalytic activity. However, traditional doping method using ammonia as N source is still unsatisfactory because of its safety risks and high cost. In this work, a benign and cost-effective method has been developed to prepare the B/N co-doped TiO2. First, we use TiB2 as the raw material to prepare B-doped TiO2 via a hydrothermal method followed by heat treatment. Then the B/N co-doped TiO2 can be simply produced by annealing the mixture of the as-prepared B-doped TiO2 and urea. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results show that the samples present anatase structure and spherical morphology. X-ray photoelectron spectroscopy (XPS) analysis indicates that B and N atoms exist not only on the surface but also in the bulk. Moreover, the content of N atoms decreases with the depth in the sample while Ti3+ content gradually increases with the depth. UV-vis absorption spectrum shows an extension of light absorption into the full visible region. More importantly, the B/N co-doped TiO2 exhibits promoted photocatalytic activity under both UV and visible light than the B-doped TiO2. Keywords: TiO2, B/N co-doping, full visible light absorption, photocatalysis.

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1. Introduction Semiconductor-based photocatalysts (such as anatase TiO2) have attracted great attention because of the potential applications in water splitting and dye-sensitized solar cells. 1-8 However, due to the large band gap of TiO2 (Eg=3.2 eV for anatase TiO2, and Eg=3.0 eV for rutile TiO2), undoped TiO2 can only work under the UV light (about 5% of solar light). The low photocatalytic efficiency makes it difficult or even nearly impossible to be widely used.9-15 Therefore, it is important for TiO2 to achieve photocatalytic response in a wider range extending to visible light. Great efforts have been made to modify TiO2 by doping metals or nonmetals in order to generate impurity states, narrow the band gap, and extend the photoresponse from UV to visible light region. Since Asahi successfully prepared N-doped TiO2 by sputtering method, 1 anion (such as C, B, N, F, S) doping (single doped and co-doped) has been extensively explored to narrow the band gap of TiO2.16-22 Both the experimental results and theoretical analysis have showed that, B/N co-doped TiO2 exhibits the best visible light absorption and photocatalytic activity.3, 6, 23-25 Because [BO4] unit is formed in the TiO2 lattice, this kind of structure can effectively weaken nearby Ti-O bonds for easy substitution of N3- for lattice O2- by contributing its extra electrons, which also compensates for the charge difference between N3- and O2- .6 This kind of elements can introduce impurity states into the band gap of TiO2, which is the key to obtaining visible light absorption and response. With boric acid as the B source and ammonia as the N source, TiO2 gel was prepared by sol-gel method and annealed in air atmosphere to achieve a B/N-doped anatase TiO2.3, 17 Unfortunately,

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this kind of doping always results in a large number of anions concentrating on the surface, which is unable to narrow the band gap or extend the photoresponse. Instead, it always produces recombination centers during the photocatalytic reaction.26-29 In order to solve this problem. Liu6 et al used TiB2 as the precursor via a hydrothermal method to synthesize interstitial B-doped TiO2. Then, the as-prepared interstitial B-doped TiO2 was annealed in ammonia atmosphere. B-N co-doped red anatase TiO2 was obtained with a band gap of 1.94 eV.18 This red TiO2 showed great visible light absorption and photocatalytic activity. Even though ammonia is an ideal choice for the TiO2 N doping, some drawbacks such as high cost and safety risks still remain, which makes it difficult for mass production. So developing a benign and cost-effective way to synthesize B/N co-doped TiO2 is of great significance. In this research, we used TiB2 as the precursor and urea instead of ammonia as the N source. Then hydrothermal method and heat treatment were employed to synthesize B-doped TiO2. After that, the B/N co-doped TiO2 was successfully prepared by simply mixing the as-prepared B-doped TiO2 with urea and annealed in a tube furnace. During this process, urea decomposed to ammonia at high temperatures and in situ N doping could be achieved. The obtained B/N co-doped anatase TiO2 can effectively absorb light from the UV region to the whole visible light region and shows promoted photocatalysis activity for methyl orange degradation. Compared with other methods, this new method to prepare B/N co-doped anatase TiO2 is simple and affordable. 2. Experimental

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Synthesis of B-doped TiO2: In a typical synthesis procedure, 0.275 g of TiB2 (98% purity, particle size: 4-8 μm, Aladdin) was added to 60 ml mixed solution containing 0.11 mol/L sodium sulfate (99.0% purity, AR grade, Tianjin, ShengAo Company ) and 1 mol/L hydrochloric acid. The mixture was then transferred to a Teflon lined autoclave and heated up to 200°C in an oven for 24 hrs. The resulting product was centrifuged, and washed with deionized water for three times to remove soluble impurities. The centrifuged sample was dried at 60°C in air atmosphere. Finally, the dried sample was placed in a muffle furnace in air atmosphere at 600°C for 2 hrs (heating rate 5°C / min), then cooled to room temperature naturally. Synthesis of B/N co-doped TiO2: The sample prepared by hydrothermal synthesis was mixed with urea (99.0% purity, AR grade, Tianjin, HaiJing Company), placed in a tube furnace under vacuum conditions (10-1Pa) with a heating rate of 5°C / min to 600°C for 60 min. Then the obtained powder was washed three times with deionized water and centrifuged to remove soluble impurities, and finally dried for 24 hrs at 60°C to obtain B/N co-doped TiO2. During these processes, urea was heated for the following reaction to occur: ∆ → CO( NH 2 ) 2  NH 3 ↑ + HCNO

(1)

Preparation of the samples for catalytic activity evaluation38: 1.0 g as-prepared TiO2 and 0.186 ml ethylene glycol solution were added to 10 ml ethanol, and magnetically stirred for 10min and then ultrasonically cleaned for 5min. After irradiation with UV light for 20 min, the sample was dried at 50°C and finally collected.

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Photocatalytic activity was conducted under UV (30 W) or visible light (Xe: 300 W) irradiation to degrade methyl orange. The photocatalyst (0.1 g) was dispersed in 50 ml methyl orange solution (10 mg/L). The suspension was ultrasonically treated and stirred in the dark for 30 min before irradiation. During the degradation process, 5 ml of the reaction solution was taken out every 30 min. Methyl orange adsorption spectra were obtained using a UV-vis spectrophotometer at 466 nm after removing the photocatalysts from the suspension. The crystal structure of the synthesized TiO2 powders was analyzed by X-ray diffraction (Shimadzu Limited, XRD-7000). The crystallite sizes and morphologies of the TiO2 powders were observed using a field emission scanning electron microscope (JEOL, JSM-6700F). The chemical compositions of the samples were investigated by X-ray photoelectron spectroscopy (KRATOS, AXIS ultra DLD). UV-vis absorption spectra of TiO2 powders were obtained using a UV-visible spectrophotometer (PE, Lambda 950). 3. Results and discussion 3.1 Phase analysis Figure 1 shows the X-ray diffraction pattern of the sample. After the hydrothermal reaction for 24 hrs, the characteristic diffraction peaks of the precursor TiB2 disappear and all the diffraction peaks belong to the TiO2 anatase phase. The diffraction peak changes indicate that TiB2 has transformed into anatase TiO2 after the hydrothermal reaction. Meanwhile, the following annealing and N-doping process do not change the crystal structure of the TiO2, and the anatase phase persists.

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Figure 1 X-ray diffraction patterns of the samples during different stages (a) TiB2, (b) after hydrothermal treatment, (c) after heat treatment, (d) B/N co-doped 3.2 Color and microstructure analysis Figure 2 shows the SEM images and photographs of the samples at different stages. Figure 2(a) is the SEM image of the precursor TiB2, which is mostly hexagonal flake crystal with the size of 4 to 8 μm. Figure 2(b) shows that most TiO2 particles have well developed spherical morphology, the surfaces of which are occupied by small octahedral particles with an average size between 200 and 500 nm. According to the XRD results (Figure S1, Supporting Information), these small regular octahedral particles correspond to anatase TiO2 particles. This result infers that TiB2 particles may gradually dissolve from the outside to the inside during the hydrothermal reaction, while TiO2 continuously nucleates and grows around TiB2. The process is similar to Ostwald ripening (Figure S2, Supporting Information).7, 30 As shown in Figure 2(c) and Figure 2(d), the morphology, crystal phase and size of these microspheres show no obvious change after calcinations at 600°C in air for 2 hrs and after N-doping. More interestingly, the color of the sample in each stage shows significant changes (Figure 2(e)). The precursor TiB2 and the sample after 7



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hydrothermal treatment show dark grey, after calcinations at 600°C in air for 2 hrs the sample is grey, finally, after N-doping the sample is brick red. This result suggests that the color change is mainly related to the impurity element added. When the B, N atoms incorporated TiO2, impurity states will be introduced into the band gap of TiO2. When the energy of the incident light is greater than or equal to the intrinsic band gap of TiO2, the valence band of TiO2 and impurity states will simultaneously excite electrons, resulting in more photo-generated electrons and holes than intrinsic TiO2. These photo-generated electrons and holes are called photocarriers for short. When the energy of the incident light is smaller than the band gap energy of the intrinsic TiO2, it is hard to directly generate electrons from the valence band to conduction band of TiO2, but it will excite electrons on impurity states into the conduction band to produce photo-generated carriers. In this case, the amount of photo-generated carriers entirely depends on the content of impurities in TiO2.

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Figure 2 SEM and optical images of the samples at different stages. (a) TiB2, (b) after hydrothermal reaction, (c) after heat treatment, (d) B/N co-doped, (e) optical images of the sample at different stages, Ⅰ: TiB2, Ⅱ: after hydrothermal treatment, Ⅲ: after heat treatment, Ⅳ: B/N co-doped 3.3 XPS depth analysis of N1s and N elements Figure 3 are the XPS depth analysis of N1s and N elements of B/N co-doped TiO2. As shown in Figure 3(a), a strong peak at 399.6 eV and a weak shoulder peak at

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400.3 eV appear in the N1s XPS spectrum of the sample after N-doping. The former originates from the substitution of nitrogen for lattice oxygen by forming Ti-N bonds or O-Ti-N bonds, and the latter is from interstitial nitrogen to form Ti-O-N bonds.6, 31-33

Moreover, Figure 3(b) shows that the N elements can still be detected with Ar+

sputtering from 4-40 s, and the molar percentage of N elements is reduced from 2.74% to 1.85%. This means that the N element exists not only on the surface but also in the bulk of the sample, and the content decreases with the depth in the sample.

Figure 3 XPS depth analysis of N1s and N elements of B/N co-doped TiO2. (a) XPS spectrum of N1s; (b) depth distribution of N elements 3.4 XPS depth analysis of Ti2p and B1s Figure 4 are the XPS depth analysis of Ti2p and B1s of B/N co-doped TiO2. As can be seen from Figure 4(a), a weak peak in the low binding energy position (457.8 eV) of Ti2p gradually grows with the Ar+ sputtering time. In addition, the peak gradually strengthens with the depth into the sample. According to the previous reports, this weak peak is believed to result from Ti3+.6, 35-37 After B doping TiO2, Ti3+ would be formed on the surface of the sample. Therefore, the photocatalytic activities

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of B-doped TiO2 under ultraviolet and visible light are determined by both interstitial atoms B and Ti3+ formed on the surface. 7 For B/N co-doped TiO2, after N atoms substituting O atoms in the lattice, the existence of B atoms and the different electrons in the outer layer of N, O atoms will result in reducing the number of Ti3+ on the surface. 6 Meanwhile, the introduction of N atoms will directly introduce the impurity states (states located at the top of the valence band) in the band gap of TiO2. So this result implies that the content of Ti3+ gradually increases with the depth. More importantly, no Ti3+ is detected on the surface. This result is different from the previous N-doping of TiO2, which often produces Ti3+ on the surface of the particles.6, 7, 23-25, 34

The gradient distribution may be caused by B-doping since B1s is also

detected. In Figure 4(b), the binding energy of B1s appears at 191.6 eV-193.3 eV, in which the 191.6 eV peak corresponds to the B-N on the surface and the 193.3 eV peak corresponds to B2O3. 17

Figure 4 XPS depth analysis of Ti2p (a) and B1s (b) spectrum of B/N co-doped TiO2. 3.5 B/N co-doping mechanism analysis B/N co-doping mechanism can be understood according to the above results as 11



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shown in Figure 5. For B doped TiO2, TiB2 is dissolved and produces TiO2 during the hydrothermal reaction; meanwhile, TiO2 continuously nucleates and grows, coating the residual TiB2. Finally, the Ti-B bond of residual TiB2 is breached by heat treatment to achieve B doping.7 For B/N co-doping, the resulting B-doped TiO2 were mixed with urea particles and then annealed. Urea decomposed to produce ammonia when the temperature was higher than 160°C, and ammonia reacted with the B-doped TiO2 to achieve B/N co-doping. From the XPS data, B-N bond was detected on the surface of B/N co-doped sample. Different from conventional N-doped TiO2, the N-doped TiO2 in this work is carried out by first mixing the B-doped TiO2 particles with urea and then annealing. The resultant N element has a gradient distribution in the TiO2 particles, as shown in Figure 5(a). The reason is that B-N bond is formed on the surface, which can not only weaken the Ti-O bond but also compensate an extra electron of the outer layer after N atoms replacing O atoms, to avoid the formation of oxygen vacancies and Ti3+ on the surface after the incorporation of N atom.6,7 There are two possible formations of B-N bond, one is nitrogen substitutes oxygen in the Ti-O-B structure to form Ti-N-B structure; the other is Ti-O-B structure reacts with the exposed boron atom during nitriding reaction to form Ti-O-B-N structure. 6, 25The impact on the atomic structure is shown in the upper right corner of Figure 5(b). The process can be expressed by the following defect equations: 18 B + Ti 4 + → 1/s B s + + Ti 3+ Ti 3+ + O 2− + N → Ti 4+ + O + N 3−

(2) (3)

As the depth increases, B atoms and N atoms decrease and Ti3+ gradually forms.

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When there are no B atoms, N atoms substitute O atoms and form Ti3+ due to the different number of electrons in the outermost layer. This kind of atomic structure can be shown in the lower right corner of Figure 5(b). The formation of Ti3+ can be expressed by the equation: Ti 4+ + e − → Ti 3+

(4)

Figure 5 Schematic of B/N co-doping mechanism. (a) Schematic of doping process; (b) the illustration of doping mechanism 3.6 UV - visible absorption analysis Figure 6 shows the UV-vis absorption spectra of the samples. Curve (a) exhibits the absorption band edge of the white B-doped TiO2 sample at about 400nm, curve (b) shows the absorption band edge of red B/N co-doped TiO2 at around 700nm, basically covering the full visible light spectrum, indicating that the incorporation of N elements in TiO2 lattice greatly improves the visible light absorption. In previous studies, the absorption band edge of N-doped anatase TiO2 was around 400 to 550nm.1, 14 In this study, B-doped TiO2 does not show visible light absorption, which is consistent with the results of previous studies.3,

6, 23-25

When N atoms are

incorporated into B-doped TiO2, the absorption spectrum of the sample red shifts to

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the visible light region and covers the full visible light spectrum, as shown from curve (b) of Figure 6.

Figure 6 Ultraviolet visible diffuse reflectance spectroscopy of the sample (a) B-doped TiO2, (b) B/N co-doped TiO2 3.7 Photocatalytic properties of the samples Figure 7 shows the photocatalytic efficiency curves of the B-doped and B/N co-doped samples under UV-light (Figure (a)) and visible light (Figure (b)). As can be seen, the photocatalytic efficiency of B/N-doped TiO2 is higher than that of B-doped TiO2 under both lights. For B-doped TiO2, as demonstrated above, Ti3+ is produced after B doping. Therefore, the interaction between impurity element B and Ti3+ causes photoresponse under both UV-visible and visible light. For B/N co-doped TiO2, B-N bonds are formed on the surface, which effectively inhibits the formation of Ti3+. In addition, the interaction between impurity elements B and N results in the photoresponse under both UV-visible and visible light. More importantly, because of the remarkable absorption increase in the UV and visible light region of B/N co-doped TiO2, the B/N co-doped sample can acquire more photo-generated carriers (electron-holes) compared to B-doped samples under the same light irradiation, and 14


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these photo-generated carriers can be directly involved in the photocatalytic reaction. Figure 7 Degradation of methyl orange of B-doped and B/N co-doped TiO2 at

different times. (a) UV light, (b) visible light Ⅰ: B-doped TiO2, Ⅱ: B/N co-doped TiO2 4. Conclusion In this study, we introduced a new approach to prepare B/N co-doped anatase TiO2. B-doped TiO2 was synthesized using TiB2 as a precursor through hydrothermal process and heat treatment. The obtained sample was mixed with urea and annealed. After decomposition of ammonia at high temperatures, B/N co-doped anatase TiO2 was obtained. B atoms stayed on the surface of the sample, and the content of N atoms gradually decreased with the depth of the sample, while Ti3+ gradually increased. Compared with the B-doped TiO2, B/N co-doped TiO2 showed strong absorption in the visible region, and the absorption spectrum covered the full visible light spectrum. The prepared B/N co-doped sample showed better photocatalytic properties under UV and visible light. This work provides a more reliable and affordable method to prepare B/N co-doped TiO2.

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Acknowledgements We are grateful for the financial support by the Education Department of Shaanxi Province of P.R China (Grant no. 12JK442). We also thank State Key Laboratory for Mechanical Behavior of Materials for financial support. Supporting Information Available X-ray diffraction patterns, SEM images of the samples at different stages. This information is available free of charge via the Internet at http://pubs.acs.org References [1] Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y., Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. J. Science. 2001, 293, 269−271 [2] Feng, N. D.; Wang, Q.; Zheng, A. M.; Zhang, Z. F.; Fan, J. S.; Liu, B.; Amoureux, J. P.; Deng, F., Understanding the High Photocatalytic Activity of (B, Ag)-Codoped TiO2 under Solar-Light Irradiation with XPS, Solid-State NMR, and DFT Calculations. J. Am. Chem. Soc. 2013, 135, 1607−1616 [3] Czoska, A. M.; Livraghi, S. M.; Paganini, C.; Giamello, E.; Valentin,C. D.; Pacchionib, G., The Nitrogen–Boron Paramagnetic Center in Visible Light Sensitized N-B Co-doped TiO2 Experimental and Theoretical Characterization. J. Phys. Chem. Chem. Phys. 2011, 13, 136–143 [4] Chen, X. B.; Liu, L.; Peter Y.; Samuel, S. M., Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. J. Science. 2011, 746, 331 [5] Liu, G.; Sun, C. H.; Wang, L. Z.; Smith, S. C.; Lu,

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N Codoped Anatase TiO2 - The Journal

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