过渡金属与N共掺杂锐钛矿TiO_2光学性质研究
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摘要
TiO_2化学性质稳定,具有无毒,高效用,便宜等特点,自从许多环境污染物在TiO_2表面被氧化降解后,已被广泛应用于水和空气的净化领域.当二氧化钛的能带受到光照后,能级中的电子获得能量跃迁到导带,而空穴停留在价带.电子和空穴能够促使在TiO_2表面发生还原反应.污染物被分解成二氧化碳和水.然而,由于TiO_2的宽带隙(3.3eV)和特征峰吸收处于紫外区,紫外光在太阳光中只有2%-3%的份额,这限制了TiO_2的广泛应用.因此,为了最大限度的利用太阳能,最大限度的使其的特征吸收峰红移,科学界为之付出了不懈的努力.过渡金属掺杂能够使TiO_2的光学吸收系数红移至可见光区.由于其半填充的离子构型,通过形成新的中间能级,从而降低TiO_2的能带.虽然过渡金属改性的TiO_2的光催化活性在可见光区能得到提高,但对实际应用它的效率较低.众所周知,氮掺杂TiO_2对其的光学性质有影响,并且有助于其光催化性能的提高.本文对氮和过渡金属共掺杂TiO_2的光学性质进行了研究.
本文采用第一性原理密度泛函理论超软赝势平面波方法,对N和过渡金属元素R(R=钒、铬、锰、铁、钴、镍、铜、锌)共掺杂锐钛矿TiO_2的电子结构、二次差分电荷密度、总电荷密度、偶极矩及光学性质进行了研究.研究结果显示,在可见光区域,N和Cr共掺杂锐钛矿TiO_2的光学吸收系数与实验值相符.计算结果表明,在可见光区,N和过渡金属元素共掺杂锐钛矿TiO_2有助于光学吸收系数的提高.在八种计算模型中,N和Mn共掺杂TiO_2在可见光区具有最高的光学吸收系数.在可见光区,离子键有助于增强TiO_2的光催化作用.总之,对于掺杂后的TiO_2,TiO_6八面体具有大偶极矩将增强TiO_2在可见光区的光响应,但是,在本文的研究中,发现TiO_6八面体具有小偶极矩反而会增强改性TiO_2在可见光区的光学吸收系数.
Titanium dioxide, as a chemically statble, nontoxic, highly effecient, and relatively inexpensive phoyocatalyst, has been widely applied for water and air purification since many evironmental pollutants can be degraded by oxidation and reduction processes on TiO2surface. Under irradiation with light of sufficient energy to span TiO2's band gap, the electrons can be promoted into the conduction band (CB), leaving a hole (h+) in the valence band (VB). Then, electrons and holes can initiate redox reactions on the surface of TiO2. Finally, pollutants can be decomposed into CO2and H2O. However, the application of TiO2is limited by its UV activation requirement because of its large band gap (3.3eV). Therefore, efforts have been directed towards shifting the optical sensitivity of TiO2from UV to the visible-light region for the efficient use of solar energy, which is composed of only about2-3%UV light. Transition metal dopants have been widely used to extend TiO2's light absorption into the visible-light region. Iron is frequently empolyed owing to its unique half-filled electronic configuration, which might narrow the energy gap through the formation of new intermediate energy levels. Although transition metal modified TiO2photocatalysts are active under visible-light irradiation, the efficiency is still low for practical use. To our best knowledge, N doped TiO2have an effect impact on the optical properties of TiO2, and been helpful for enhancing the TiO2photocatalysts activity. In this work, the optical properties of N and transition metals codoped TiO2can be studied.
The electronic structures, def-ormation charge density, dipole moment and optical properties of N and transition metal R (R=V, Cr, Mn, Fe, Co, Ni, Cu and Zn) codoped anatase TiO2are studied using the plane-wave ultrasoft pseudopotential method of density functional theory (DFT). The results exhibit that the absorption coefficients of the N+Cr-doped TiO2is in accordance with the experimental values under visible-light region. The calculated results reveal that the N+R-doped TiO2has helpful for enhancing the absorption coefficient under visible-light region, especially, among the eight models, N+Mn-doped TiO2has a largest value of absorption coefficient under visible-light region. The ionic bonding would contribute to increase the photocatalytic activity of TiO2h under visible-light region. All by all, for doped TiO2, the large dipole moment of TiO6octahedron can enhance the optical responses of visible-light, but in this work, it is found that the small dipole moment of TiO6octahedron would increase the absorption coefficient under visible-light region.
本文采用第一性原理密度泛函理论超软赝势平面波方法,对N和过渡金属元素R(R=钒、铬、锰、铁、钴、镍、铜、锌)共掺杂锐钛矿TiO_2的电子结构、二次差分电荷密度、总电荷密度、偶极矩及光学性质进行了研究.研究结果显示,在可见光区域,N和Cr共掺杂锐钛矿TiO_2的光学吸收系数与实验值相符.计算结果表明,在可见光区,N和过渡金属元素共掺杂锐钛矿TiO_2有助于光学吸收系数的提高.在八种计算模型中,N和Mn共掺杂TiO_2在可见光区具有最高的光学吸收系数.在可见光区,离子键有助于增强TiO_2的光催化作用.总之,对于掺杂后的TiO_2,TiO_6八面体具有大偶极矩将增强TiO_2在可见光区的光响应,但是,在本文的研究中,发现TiO_6八面体具有小偶极矩反而会增强改性TiO_2在可见光区的光学吸收系数.
Titanium dioxide, as a chemically statble, nontoxic, highly effecient, and relatively inexpensive phoyocatalyst, has been widely applied for water and air purification since many evironmental pollutants can be degraded by oxidation and reduction processes on TiO2surface. Under irradiation with light of sufficient energy to span TiO2's band gap, the electrons can be promoted into the conduction band (CB), leaving a hole (h+) in the valence band (VB). Then, electrons and holes can initiate redox reactions on the surface of TiO2. Finally, pollutants can be decomposed into CO2and H2O. However, the application of TiO2is limited by its UV activation requirement because of its large band gap (3.3eV). Therefore, efforts have been directed towards shifting the optical sensitivity of TiO2from UV to the visible-light region for the efficient use of solar energy, which is composed of only about2-3%UV light. Transition metal dopants have been widely used to extend TiO2's light absorption into the visible-light region. Iron is frequently empolyed owing to its unique half-filled electronic configuration, which might narrow the energy gap through the formation of new intermediate energy levels. Although transition metal modified TiO2photocatalysts are active under visible-light irradiation, the efficiency is still low for practical use. To our best knowledge, N doped TiO2have an effect impact on the optical properties of TiO2, and been helpful for enhancing the TiO2photocatalysts activity. In this work, the optical properties of N and transition metals codoped TiO2can be studied.
The electronic structures, def-ormation charge density, dipole moment and optical properties of N and transition metal R (R=V, Cr, Mn, Fe, Co, Ni, Cu and Zn) codoped anatase TiO2are studied using the plane-wave ultrasoft pseudopotential method of density functional theory (DFT). The results exhibit that the absorption coefficients of the N+Cr-doped TiO2is in accordance with the experimental values under visible-light region. The calculated results reveal that the N+R-doped TiO2has helpful for enhancing the absorption coefficient under visible-light region, especially, among the eight models, N+Mn-doped TiO2has a largest value of absorption coefficient under visible-light region. The ionic bonding would contribute to increase the photocatalytic activity of TiO2h under visible-light region. All by all, for doped TiO2, the large dipole moment of TiO6octahedron can enhance the optical responses of visible-light, but in this work, it is found that the small dipole moment of TiO6octahedron would increase the absorption coefficient under visible-light region.
引文
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[3]Shu Y, Masakazu K, Zhang QW, et al. Synthesis of Visible-light Responsive nitrogen/carbon doped titania photocatalyst by mechanochemical Doping [J]. Mater. Sci.2007,42:2399-2404.
[4]Venkatachalam N, Palanichamy M, Murugesan V, et al. Alkaline Earth Metal Doped Nanoporous TiO2for Ehanced Photocatalytic Mineralisation of Bisphenol-A [J]. Catalysis Communications,2007,8:1088-1093.
[5]Asahi R, Morikawa T, Ohwaki T, et al. Visible-light Photocatalysis in Nitrogen-doped Titanium oxides[J]. Science,2001,293(13):269-271.
[6]Sakthivel S, Kisch H, Daylight Photocatalysis by Carbon-modified Titanium Dioxide[J]. Angew. Chem. Int. Edit.,2003,42(40):4908-4911.
[7]Wang J S, Yin S, Zhang Q W, et al. Mechanochemical Synthesis of Sr-TiO3-xFx with High Visible Light Photocatalytic Activities for Nitrogen Monoxide Destruction[J]. Materials Chemistry,2003,13(9):2348-2352.
[8]Ohnao T, Akiyaoshi M, Umebayashi T, et al. Preparation of S-doped TiO2Photocatalysts and Their Photocatalytic Activity under Visible Light[J]. Appli. Cataly. A General.,2004,265(1):115-121.
[9]Yang X X, Cao C D, Erickson L, et al. Photo-Catalytic degradation of Rhodamine B on C-, S-, N-, and Fe-doped TiO2under visible-light irradiation[J]. Applied Catalysis B: Environmental.2009,91:657-662.
[11]李晓辉,刘守新.N,F共掺杂TiO2可见光响应光催化剂的酸催化水解法制备及表征[J].物理化学学报.2008,24：2019—2024.
[12]韦存福.微波辅助N,S共掺杂纳米TiO2制备,表征及光催化性能研究[J].2008年重庆大学硕士学位论文.
[13]丁卉,张诺,戎非,付德刚.氮氟共掺杂二氧化钛薄膜的制备,表征及杀菌性能[J].无机材料学报.2011,26：517—522.
[14]李辉,王金淑,李洪利,Yin Shu, Sato Tsugio氮硫掺杂介孔TiO2薄膜结构及其光催化性能[J].无机材料学报.2009,24：909-914.
[15]吴佐林,李道荣,胡长文,尹棋亚.N—C改性纳米TiO2可见光催化剂的制备与表征[J].高新技术.2007,14：1—2.
[16]顾德恩,杨邦朝,胡永达.非金属元素掺杂TiO2的可见光催化活性研究进展[J].功能材料.2008,39(1)：1—5.
[17]Asahi R, Morikawa T, Ohwaki T, et al. Visible-light photocatalysis in nitrogen-doped titanium oxides[J]. Science,2001,293(5528):269-271.
[18]Khan S U M, Al-Shahry M, Ingler J r W B. Efficient photochemical water splitting by a chemically modified n-TiO2[J]. Science,2002,297:2243-2245.
[19]Zhang renhui, Wang qing, Li qiang, Dai jianfeng, Huang duohui. First-principles calculations on optical properties of C/N-doped and C-N-codoped anatase TiO2Physica B.2011,406:3417-3422.
[20]唐玉朝黄显怀俞汉青胡春.N掺杂TiO2光催化剂的制备及其可见光活性研究[J].无机化学学报.2005,11：1747—1751.
[21]刘会景,柏源,孙红旗,金万勤.掺杂基团对氮改性TiO2紫外光催化活性影响的机理研究[J].无机材料学报.2009,24：443—447.
[22]唐建军,王岳俊,邓爱华,袁辉洲,周康根.N掺杂TiO2光催化剂的制备与表征[J].2007,7：1555—1560.
[23]冯彩霞,王岩,金振声,张顺利.N掺杂纳米TiO2可见光催化氧化丙烯的动力学行为[J].物理化学学报.2008,24(4)：633—638.
[24]Yamashita H, Ichihashi, Takeuchi M, Kisshiguchi, Anpo M, Characterization of metal ion-implanted titanium oxide photocatalysts operating under visible light irradiation[J]. J. Synchroton Rad.1999,6:451-452.
[25]Tsutomu Umebayashi, Tetsuya Yamaki, Hisayoshi Itoh, Keisuke Asai. Analysis of electronic structures of3d transition metal-doped TiO2based on band calculations[J]. Journal of Physics and Chemistry of Solids.2002,63:1909-1920.
[26]Yin J B, Zhao X P. Enhanced Electrorheological Activity of Mesoporous Cr-Doped TiO2from Activated Pore Wall and High Surface Area[J]. J. Phys. Chem. B,2006,110(26):12916-12925.
[27]Rebecca Janisch, Priya Gopal, Nicola A Spaldin. Transition metal-doped and ZnO—present status of the field[J]. Journal of Physics:Condensed Matter.2005,17:657.
[28]Choi W Y, Termin A, Hoffmann M R, The role of metal ion dopants in quantum-sized TiO2:Correlation between photoreactivity and charge carrier recombination dynamics[J]. J. Phys. Chem.1994,98:13669-13679.
[29]SHI Zai-Feng,SONG Xue-Fang,LI Juan. Synthesis of Fe3+-doped Nanosized TiO2Photocatalyst with Sol-gel Based Self-propagating High-temperature Synthesis Process [J]. Chinese Journal of Applied Chemistry.2010,27:1171-1188.
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