The Effect of Cu Doping on the Transformation from Rutile to Anatase and Cu Occupation Tendency in TiO 2 Solid Solution

TiO2 doped with different amounts of Cu ions (from 0 to 3mol%) was synthesized by sol-gel method. .e samples were characterized by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR)..e XRD analysis showed that the Cu-doped TiO2 samples exhibit anatase and rutile phases..e lattice parameters remain unchanged, independent of Cu content. Diameter of TiO2 increased significantly with increasing concentrations of Cu. .e investigated results indicate that a greater portion of the Cu ions are well incorporated into the anatase and rutile TiO2 lattices. .e stretching vibration frequencies of the interatomic bonds were calculated by the electronegativity principle..e calculated data were compared with infrared spectra..e results show that in the rutile and anatase phases, O atoms in the TiO2 lattice and some interstitial Cu atoms form Cu-O bond, and other substitutional Cu that replaces Ti atoms in TiO2 lattice form Cu-O bond with O atoms in the TiO2 lattice.


Introduction
Titanium dioxide (TiO 2 ) is widely concerned with its cheapness, stability, environmental friendliness, and photocatalytic properties [1].TiO 2 application is encircled as a photocatalyst in heterogeneous catalysis, or the production of hydrogen and electric energy in solar cells, as white pigments for cosmetic and paint industries, as a gas transducer, as a sunscreen cosmetic, as an edible pigment, in electronic equipment, in ceramics, and others [2].However, undesired recombination of photoexcited carriers and wide band gap (3.2 eV) severely limits its practical application.One key challenge is to develop catalysts with high catalytic capability.Doping modification is found to play an important role for the catalytic performance of TiO 2 .e photocatalytic activity of TiO 2 could be obviously improved by doping Cu, N, S, Fe, C, etc. [3].In particular, doping of metals seems to be an effective way.Another approach to change the physical, optical, structural, and photocatalytic properties of titania includes an employment of d-block metal ions (zinc, zirconium, iron, chromium, nickel, vanadium, or copper) [4].
Recently, Cu doping has been increasingly investigated as a dopant for titania.
e origin of the ferromagnetic property was explained based on the concentration of oxygen vacancies increased by Cu doping [5].From the structural and surface analysis of the catalysts, we have stated that the occurrence of highly disperse and reducible Cu 2+ species is directly related to the photocatalytic activity for the H 2 production reaction [6].At present, we have no information about the doping position of Cu 2+ ions that were researched [7].Cu could replace Ti in the substitutional sites or be incorporated in the interstitial sites.In some cases, they may segregate on the surface [8].e doping position of Cu can be calculated by the electronegativity principle [9].Different doping positions of Cu atoms have an effect on the properties of particle, electron structure, and light absorption.In conclusion, there is a need to analyze the doped position of Cu, which will help understand in detail the role of the dopant in altering TiO 2 properties [10].
In this work, Cu-doped and undoped TiO 2 nanomaterials were prepared with the sol-gel method.e crystalline phase and IR spectra of the samples were characterized with X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR).
e objectives/goals of this study were to illustrate the Cu doping position that could be simulated by the electronegativity principle.

Sample Preparation.
e TiO 2 system was prepared by a sol-gel method using tetrabutyl titanate as a precursor (10 ml TBT) in water/ethanol solution (200 ml ethanol and 2 ml H 2 O).Forced hydrolysis of the TBTsolution was achieved by adding certain volume of bidistilled water (8.4 ml).e solgel synthesis in acidic solution was performed by substituting the initial 2 ml of water by the same volume of nitric acid (1 M).On the other hand, the copper-doped systems were obtaining by adding the 25 ml of Cu(NO 3 ) 2 •3H 2 O. Doped and undoped TiO 2 systems were calcined in air at 550 °C for 3∼5 h.

Experiment Principles.
e IR (photons or energy) absorption of molecules causes the vibration of each chemical bond in the molecule.
e bond vibrations are similar to diatomic vibrations [11].e reduced mass μ can be expressed as follows: where m 1 and m 2 are the masses of the two bonded atoms (m Ti � 47.87; m Cu � 55.845; m O � 16.00).e most mature theory for the vibrational ground state IR model is the harmonic oscillator model.According to classical mechanics, the stretching force constant k and frequency υ satisfy the following relation [12]: In Equation ( 2), the unit of υ is cm −1 .At the same time, Yang et al. [13] proposed the following relationship between the force constants and electronegativity: where k is the stretching force constant, d is the bond length, N is the bond order, and Xa and Xb are the electronegativities of the atoms at both ends of the bond (k unit: dynes/ cm10−5; d unit: Å). e values of m and n are 1.67 and 0.30, respectively (for stable molecules and stably covalent atoms) [14].e bond order N can be calculated as follows: N � (total number of electrons in a stable structure-total number of valence electrons)/2; the calculation yields a bond order of 0.5.By looking up the electronegativity table, the following values are obtained: XTi � 1.54, XO � 3.44, and XCu � 1.90 [14].

TiO 2 Molecular Structure Model.
Figure 1 is the structural model of TiO 2 .From the periodic arrangement, it can be seen that it constitutes multiple oxygen octahedral structure units, and so only the model of a single cell structure needs to be discussed.e basic unit in the structure of both rutile and anatase TiO 2 is oxygen octahedral.
e subscript of element symbol represents the position of this atom.e number between the two atoms represents the bond length between the two atoms.For instance, Ti A representing Ti atom is in titanium lattices of anatase TiO 2 .e lower right corner of the symbol indicated the atomic location number.e numbers between two atoms indicated the bond distance between two atoms (unit: Å). e lattice parameters of samples calculated from the XRD patterns are shown in Table 1.e lattice parameters remain unchanged, independent on Cu 2+ content. is is evident considering that a greater portion of the Cu 2+ ions is well incorporated into the anatase and rutile TiO 2 lattice.When comparing to bulk anatase and rutile TiO 2 , a small change in lattice constant has been observed for the Cu-doped TiO 2 samples as shown in Table 1.e reason for this may be due to the tensile strain in the lattice.

Infrared Spectra of Cu Solid Solution Doped TiO 2 .
Figure 4(a) indicates the infrared spectra of the Cu-doped TiO 2 samples.According to the electronegativity principle, the value of d A was substituted into formula (3), and then, two kinds of stretching vibration frequencies of Ti-O in anatase were calculated: v 1A � 622.76 cm −1 and v 2A � 511.62 cm −1 , and v A represents vibrational frequencies of the anatase.In the same way, the calculation results show that v 1R � 1108.46 cm −1 , v 2R � 1381.54 cm −1 , and v R represents vibrational frequency of the rutile.
ere were two strong absorption peaks: 3461.78 cm −1 and 1658.Journal of Spectroscopy

Conclusion
e lattice parameters of samples calculated from the XRD patterns are shown in Table 1.e lattice parameters remain unchanged, independent of Cu 2+ content. is is evident considering that a greater portion of the Cu 2+ ions was well incorporated into the anatase and rutile TiO 2 lattice.When comparing to bulk anatase and rutile TiO 2 , a small change of the lattice constant had been observed for the Cu-doped TiO 2 samples as shown in Table TiO 2 Structure Model.Figures 2(a) and 2(b) show that in the anatase, Cu replaces Ti atoms in the substitutional sites or occupy in the interstitial sites.Figures 2(c) and 2(d) show that, in the anatase, Cu replaces Ti atoms in the substitutional sites or occupy in the interstitial sites.Doping substitutional and interstitial sites were constructed by using SDD configuration, GaussView, and Gaussian09w.e information of bond length and the position of each atom are shown in Figure 2. e dashed line between Cu and O indicates that Cu can form Cu 2 O with two O atoms and can also form CuO with only an O atom.
Figure 3  can be seen that all samples show anatase and rutile phase regardless of Cu 2+ content.
74 cm −1 , which were caused by the H-Osw bond, H-Os bond, and H-Ow bond.s: Stretching vibration, w: bending vibration, Ti A -O: Ti-O bond in anatase, and Ti R -O: Ti-O bond in rutile.Cu A i -O: Cu atoms were incorporated in the interstitial anatase sites, Cu A s -O: Cu atoms replace Ti in the substitutional anatase sites, Cu R i -O: Cu atoms were incorporated in the interstitial rutile sites, and Cu R s -O: Cu atoms replace Ti in the substitutional rutile sites.e following data are obtained on the basis of the electronegativity principle.Figure 4(b) indicates that the absorption peaks of Ti A -O, Cu A s -O, and Cu A i -O were similar.When the doping amount of Cu was 3 mol%, the absorption peaks of Ti A -O were narrower.Meanwhile, the absorption peaks of Cu A i -O and Cu A s -O appear in the anatase lattice.Figure 4(c) indicates that when the doping amount of Cu was 5 mol%, the 2 Journal of Spectroscopy absorption peaks of Cu R i -O and Cu R s -O appear in the rutile lattice.Transmissivity of the Cu-O bond decreased from T 0.74 to T 0.38.Meanwhile, the absorption peaks of Ti R -O were also narrower.e results show that, in the rutile and anatase phases, a part of Cu and O atoms by interstitial solid solution to form Cu-O bond, and another part of Cu replaces Ti atoms by substitutional solid solution to form Cu-O bond with O atoms in the TiO 2 lattice.Cu A i -O: Cu atoms were incorporated in the interstitial anatase sites, Cu A s -O: Cu atoms replace Ti in the substitutional anatase sites, Cu R i -O: Cu atoms were incorporated in the interstitial rutile sites, and Cu R s -O: Cu atoms replace Ti in the substitutional rutile sites.e following data are obtained on the basis of the electronegativity principle.

Figure 4 (
b) indicates that the absorption peaks of Ti A -O, Cu A s -O, and Cu A i -O were similar.When the doping amount of Cu was 3 mol%, the absorption peaks of Ti A -O were narrower.Meanwhile, the absorption peaks of Cu A i -O and Cu A s -O appear in the anatase lattice.

Figure 4 (
c) indicates that when the doping amount of Cu was 5 mol%, the absorption peaks of Cu R i -O and Cu R s -O appear in rutile lattice.Transmissivity of the Cu-O bond decreased from T 0.74 to T 0.38.Meanwhile, the absorption peaks of Ti R -O were also narrower.e results show that, in the rutile and anatase phases, O atoms in the TiO 2 lattice and some interstitial Cu atoms form Cu-O bond, and other substitutional Cu that replaces Ti atoms in TiO 2 lattice form the Cu-O bond with O atoms in the TiO 2 lattice.

Figure 2 :Figure 3 :
Figure 2: Cu-doped TiO 2 structure model.(a) Cu replaces Ti atoms in the anatase; (b) Cu occupies in the anatase; (c) Cu replaces Ti atoms in the anatase; (d) Cu occupies in the anatase.

Figure 1 :
Figure 1: e structural model of TiO 2 : (a) the structural model of rutile and (b) the structural model of anatase.
. According to Table1, the changing of lattice constant is due to the tensile strain.It could be simulated by the principle of electronegativity that O atoms in the TiO 2 lattice and some interstitial Cu atoms form the Cu-O bond and other substitutional Cu that replaces Ti atoms in TiO 2 lattice form the Cu-O bond with O atoms in the TiO 2 lattice.