Abstract
TiO2 doped with different amounts of Cu2+ ions (from 0 to 3 mol%) was synthesized by sol-gel method. The samples were characterized by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The XRD analysis showed that the Cu-doped TiO2 samples exhibit anatase and rutile phases. The lattice parameters remain unchanged, independent of Cu2+ content. Diameter of TiO2 increased significantly with increasing concentrations of Cu2+. The investigated results indicate that a greater portion of the Cu2+ ions are well incorporated into the anatase and rutile TiO2 lattices. The stretching vibration frequencies of the interatomic bonds were calculated by the electronegativity principle. The calculated data were compared with infrared spectra. The 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.
1. Introduction
Titanium dioxide (TiO2) is widely concerned with its cheapness, stability, environmental friendliness, and photocatalytic properties [1]. TiO2 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 TiO2. The photocatalytic activity of TiO2 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. The 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 Cu2+ species is directly related to the photocatalytic activity for the H2 production reaction [6]. At present, we have no information about the doping position of Cu2+ 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]. The 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 TiO2 properties [10].
In this work, Cu-doped and undoped TiO2 nanomaterials were prepared with the sol-gel method. The crystalline phase and IR spectra of the samples were characterized with X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR). The objectives/goals of this study were to illustrate the Cu doping position that could be simulated by the electronegativity principle.
2. Experimental Details
2.1. Sample Preparation
The TiO2 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 H2O). Forced hydrolysis of the TBT solution was achieved by adding certain volume of bidistilled water (8.4 ml). The sol-gel 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(NO3)2·3H2O. Doped and undoped TiO2 systems were calcined in air at 550°C for 3∼5 h.
2.2. Experiment Principles
The IR (photons or energy) absorption of molecules causes the vibration of each chemical bond in the molecule. The bond vibrations are similar to diatomic vibrations [11]. The reduced mass μ can be expressed as follows:where and are the masses of the two bonded atoms ( = 47.87; = 55.845; = 16.00). The 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). The values of m and n are 1.67 and 0.30, respectively (for stable molecules and stably covalent atoms) [14]. The 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].
2.3. TiO2 Molecular Structure Model
Figure 1 is the structural model of TiO2. 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. The basic unit in the structure of both rutile and anatase TiO2 is oxygen octahedral. The subscript of element symbol represents the position of this atom. The number between the two atoms represents the bond length between the two atoms. For instance, TiA representing Ti atom is in titanium lattices of anatase TiO2.
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2.4. Cu-Doped TiO2 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. The information of bond length and the position of each atom are shown in Figure 2. The dashed line between Cu and O indicates that Cu can form Cu2O with two O atoms and can also form CuO with only an O atom. The lower right corner of the symbol indicated the atomic location number. The numbers between two atoms indicated the bond distance between two atoms (unit: Å).
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2.5. Phase Analysis
Figure 3 can be seen that all samples show anatase and rutile phase regardless of Cu2+ content. The lattice parameters of samples calculated from the XRD patterns are shown in Table 1. The lattice parameters remain unchanged, independent on Cu2+ content. This is evident considering that a greater portion of the Cu2+ ions is well incorporated into the anatase and rutile TiO2 lattice. When comparing to bulk anatase and rutile TiO2, a small change in lattice constant has been observed for the Cu-doped TiO2 samples as shown in Table 1. The reason for this may be due to the tensile strain in the lattice.
2.6. Infrared Spectra of Cu Solid Solution Doped TiO2
Figure 4(a) indicates the infrared spectra of the Cu-doped TiO2 samples. According to the electronegativity principle, the value of dA was substituted into formula (3), and then, two kinds of stretching vibration frequencies of Ti-O in anatase were calculated: = 622.76 cm−1 and = 511.62 cm−1, and represents vibrational frequencies of the anatase. In the same way, the calculation results show that = 1108.46 cm−1, = 1381.54 cm−1, and represents vibrational frequency of the rutile. There were two strong absorption peaks: 3461.78 cm−1 and 1658.74 cm−1, which were caused by the H-Osw bond, H-Os bond, and H-Ow bond. s: Stretching vibration, w: bending vibration, TiA-O: Ti-O bond in anatase, and TiR-O: Ti-O bond in rutile. CuAi-O: Cu atoms were incorporated in the interstitial anatase sites, CuAs-O: Cu atoms replace Ti in the substitutional anatase sites, CuRi-O: Cu atoms were incorporated in the interstitial rutile sites, and CuRs-O: Cu atoms replace Ti in the substitutional rutile sites. The following data are obtained on the basis of the electronegativity principle. Figure 4(b) indicates that the absorption peaks of TiA-O, CuAs-O, and CuAi-O were similar. When the doping amount of Cu was 3 mol%, the absorption peaks of TiA-O were narrower. Meanwhile, the absorption peaks of CuAi-O and CuAs-O appear in the anatase lattice. Figure 4(c) indicates that when the doping amount of Cu was 5 mol%, the absorption peaks of CuRi-O and CuRs-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 TiR-O were also narrower. The 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 TiO2 lattice. CuAi-O: Cu atoms were incorporated in the interstitial anatase sites, CuAs-O: Cu atoms replace Ti in the substitutional anatase sites, CuRi-O: Cu atoms were incorporated in the interstitial rutile sites, and CuRs-O: Cu atoms replace Ti in the substitutional rutile sites. The following data are obtained on the basis of the electronegativity principle. Figure 4(b) indicates that the absorption peaks of TiA-O, CuAs-O, and CuAi-O were similar. When the doping amount of Cu was 3 mol%, the absorption peaks of TiA-O were narrower. Meanwhile, the absorption peaks of CuAi-O and CuAs-O appear in the anatase lattice. Figure 4(c) indicates that when the doping amount of Cu was 5 mol%, the absorption peaks of CuRi-O and CuRs-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 TiR-O were also narrower. The 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 the Cu-O bond with O atoms in the TiO2 lattice.
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3. Conclusion
The lattice parameters of samples calculated from the XRD patterns are shown in Table 1. The lattice parameters remain unchanged, independent of Cu2+ content. This is evident considering that a greater portion of the Cu2+ ions was well incorporated into the anatase and rutile TiO2 lattice. When comparing to bulk anatase and rutile TiO2, a small change of the lattice constant had been observed for the Cu-doped TiO2 samples as shown in Table 1. According to Table 1, 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 TiO2 lattice and some interstitial Cu atoms form the Cu-O bond and other substitutional Cu that replaces Ti atoms in TiO2 lattice form the Cu-O bond with O atoms in the TiO2 lattice.
Data Availability
The lattice parameters of the Cu-doped TiO2 samples data used to support the findings of this study have been deposited in the ICDD repository (PDF #21-1272 and PDF #21-1276). The data of vibration frequency are calculated by ourselves.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant no. 51362017) and the plan of Yunnan Province Science and Technology Commission under Grant no. 2012ZE008. This work was also financially supported by Collaborative Innovation Fund Project of Precious Metals and Advanced Materials Innovation Center (Grant no. 14051708).