Enhanced photocatalytic degradation of rutile TiO2 by analysis of the surface potential and fluorescence

Ma Liyang 1School of Materials Science and Engineering

Southwest University of Science and Technology Mianyang, China

E-mail: liyang.ma@163.com

Dong Faqin

Key Laboratory of Solid Waste Treatment and Resource Recycle (SWUST)

Ministry of Education Mianyang, China

Bian Liang Institute of Chemical Materials

China Academy of Engineering Physics Mianyang, China

Abstract—Rare earth (RE) (La3+, Ce4+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Er3+, Tm3+ and Y3+) doped rutile phase TiO2 were prepared by low temperature one step synthesis and used as photocatalyst. The RE doped rutile phase TiO2 were characterized by X-ray diffraction (XRD), Particle Size, zeta potential, UV-visible spectroscopy, fluorescence spectrum. The results show that the RE have been doped into the rutile TiO2 crystal lattice successfully.

It was analyzed that main crystal phase was rutile phase TiO2 in XRD. It shows that we can get micrometer-size rutile phase TiO2 and RE doped TiO2 by one-step method. Particle Size indicate that we can get micrometer-size rutile TiO2 and RE doped rutile TiO2 by one-step method. The results of zeta potential of RE doped rutile TiO2 explain RE how to change the photoelectric activity by increase the lattice defect. The results of FS show that RE how to influence on the electron transition and the electron trap of rutile TiO2. These results indicate that RE can enhanced photocatalytic degradation of rutile TiO2.

Keywords-rare earth; rutile TiO2; surface potentials; fluorescence effect; crystal defects

I. INTRODUCTION Heterogeneous photocatalytic degradation of organic

contaminants in wasted water on semiconductors attracts interests of many people who work in environmental sciences and engineerings in the world. TiO2 (Titanium dioxide) has been studied widely as an n-type semiconductors with excellent photocatalytic property, chemical stability and avirulence [1-5]. There are three types of TiO2 crystalline phases, at normal temperatures and pressures. They are rutile, anatase and brookite. Different crystalline phases have different band-gap and crystal texture. Therefore, they have different photocatalytic activity [6-9]. The band gap of rutile phase TiO2 (3.03ev) is broader than anatase phase TiO2 (3.20ev). So rutile phase TiO2 has higher photocatalytic activity than anatase phase TiO2. L.A.Errico et al. [10] gained a similar conclusion by studied the crystal texture and the energy bandgap of rutile TiO2 and anatase TiO2.

Rutile TiO2 has photocatalysis only under ultraviolet (UV) light [11-13]. RE (La3+, Ce4+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Er3+, Tm3+ and Y3+) were adoped to enlarged the

absorbing wavelengh of rutile TiO2. Besides, this text compared the surface potentials and fluorescence effect of undoped rutile TiO2 and RE doped rutile TiO2 with different doping ratio. This paper also discussed the photocatalytic degradation through analyzing crystal defect and surface potential.

II. EXPERIMENTAL METHOD Rutile TiO2 was produced by one-step synthesis method.

Butyl titanate (analytically pure) was used as a forerunner. Absolute ethyl alcohol was added as a co-solvent. Ammonia was added to form into Ti20O32(OH)12(H2O)18

4+.The precursor gel was obtained after centrifugation and washing with distilled water. After mixed into HNO3 Pure rutile phase TiO2 then was yielded by evaporative reflux (at 85℃, 2h, PH=2). Finally, the gel was dried at 105℃. And then, it yielded TiO2 power. RE-doped TiO2 were synthetized when added into 0.5wt%, 1.0wt%, 1.5wt%, 2.0wt% RE oxide (La2O3, CeCO3, Pr2O3, NdCl3, SmCl3, Eu2O3, Gd2O3, Tb4O7, DyF3, Er2(CO3)3, Tm2(CO3)3 and Y2O3) before evaporative reflux.

III. RESULTS AND DISCUSSION

A. XRD analysis

Conesponding author: Dong Faqin (E-mail: fqdong@swust.edu.cn Tel: 0816-6089013)

Foundation item: Sino-Kazakstan cooperation project(2006020021) and National Key Laboratory (20030200103)

Biography: Ma Liyang (1984-), Female, Master Candidate

978-1-4244-4713-8/10/$25.00 ©2010 IEEE

Figure 1. XRD patterns of pure TiO2 and RE (La3+, Ce4+, Pr3+, Nd3+, Sm3+ and Eu3+) doped rutile TiO2 (doping ratios are 0.5wt%, 1.0wt%, 1.5wt% and

2.0wt%)

Figure 2. XRD patterns of pure TiO2 and RE (Gd3+, Tb3+, Dy3+, Er3+, Tm3+ and Y3+) doped rutile TiO2 (doping ratios are 0.5wt%, 1.0wt%, 1.5wt% and

2.0wt%)

The X-ray diffraction patterns of pure rutile phase TiO2 and 0.5wt%, 1.0wt%, 1.5wt%, 2.0wt% RE-doped TiO2 were illustrated in Fig. 1 and Fig. 2. As we know that rutile structure was confirmed by(110), (101), (200), (111) and (210) diffraction peaks [14]. The XRD patterns of rutile phase TiO2 have main peak at 2θ=27.4° due to the (110) planes. Therefore rutile phase TiO2 has been detected in our characterizations. Moreover, RE oxide phase has not been found in XRD pattern, according to XRD method. From this, we can conclude that RE ions uniformly dispersed in the rutile crystallite. In the region of 20-80°, the form of crystal planes peaks of pure rutile phase TiO2 is similar to RE-doped rutile phase TiO2.

The XRD patterns of pure rutile phase TiO2 and 0.5wt%, 1.0wt%, 1.5wt%, 2.0wt% La, Ce, Y-doped rutile phase TiO2 are shown in Fig. 1 and Fig. 2. From the diffraction peaks of (110) plane at 2θ=27.4°, we can see that the more doping quantity of La, Ce-doped rutile phase TiO2 the more Crystal structure distortions. Among Y, Cs-doped rutile TiO2, it was the other way round. Thus, the dopant is expected to play a significant control in the selective crystallization of rutile phase TiO2 during one-step synthesis process.

The correlation X-ray diffraction parameters of pure rutile phase TiO2 and 0.5wt%, 1.0wt%, 1.5wt%, 2.0wt% La, Ce, Y-doped rutile phase TiO2. We can see that 1.5wt% Tb have greatest peak at 2θ=27.4°due to the (110) planes. In addition, the peak of Er lessens as the increased amount of doping. The atom radius of Ti, Er, Gd, Tb are 0.176nm, 0.226nm, 233nm and 225nm, respectively. Ti atom was replaced by RE element, so the intensity of d had changed. As the atom radius of Er smaller than Gd and Tb, that makes smaller distortion. Through the above-mentioned analysis, such a conclusion can be drawn: 1.5wt% Gd and Tb doped rutile TiO2.

B. Grain sizes analysis From the result of X-ray, we can determine the crystalline

phase of these samples. The photoelectric effect of TiO2 not only depends on crystal structure, but also is relevant with

specific surface area. The smaller particle sizes, the larger specific surface, the better photoelectric activity. Rutile TiO2 and RE doped rutile TiO2 are shown in table 1. The volume mean diameter of the coarse power is no more than 20μm, and the number average size of the power particles is less than 1μm. The reason of which is the process of Grain growth often always go with partial coacervate. It shows that we can get micrometer-size rutile phase TiO2 and RE doped TiO2 by one-step method. When added doping quantity, the crystal size enlarged. Still increase RE, and then it will exist as RE oxides. RE oxides settle onto rutile TiO2 and RE doped rutile TiO2 surfaces. This degrades the photocatalytic activity of TiO2.

TABLE I. PARTICLES SIZE OF RE (LA3+, CE4+, PR3+, ND3+, SM3+, EU3+, GD3+, TB3+, DY3+, ER3+, TM3+ AND Y3+) DOPED RUTILE TIO2

sample average volume median size(nm)

average number median size(nm)

Rutile 4913 384 0.5wt%La doped rutile TiO2 2905 252 1.0wt%La doped rutile TiO2 3014 258 1.5wt%La doped rutile TiO2 2762 215 2.0wt%La doped rutile TiO2 2975 272 0.5wt%Ce doped rutile TiO2 12185 1873 1.0wt%Ce doped rutile TiO2 2709 406 1.5wt%Ce doped rutile TiO2 10534 440 2.0wt%Ce doped rutile TiO2 11252 648 0.5wt%Pr doped rutile TiO2 12014 406 1.0wt%Pr doped rutile TiO2 12477 5723 1.5wt%Pr doped rutile TiO2 22209 9526 2.0wt%Pr doped rutile TiO2 24466 7443 0.5wt%Nd doped rutile TiO2 4873 497 1.0wt%Nd doped rutile TiO2 1250 462 1.5wt%Nd doped rutile TiO2 1432 1289 2.0wt%Nd doped rutile TiO2 29585 1178 0.5wt%Sm doped rutile TiO2 14377 5372 1.0wt%Sm doped rutile TiO2 18909 388 1.5wt%Sm doped rutile TiO2 25934 6549 2.0wt%Sm doped rutile TiO2 36648 6956 0.5wt%Eu doped rutile TiO2 20289 390 1.0wt%Eu doped rutile TiO2 31711 388 1.5wt%Eu doped rutile TiO2 18425 390 2.0wt%Eu doped rutile TiO2 25855 404 0.5wt%Gd doped rutile TiO2 15009 400 1.0wt%Gd doped rutile TiO2 11108 410 1.5wt%Gd doped rutile TiO2 17908 433 2.0wt%Gd doped rutile TiO2 21662 6701 0.5wt%Tb doped rutile TiO2 32094 445 1.0wt%Tb doped rutile TiO2 23343 397 1.5wt%Tb doped rutile TiO2 50437 395 2.0wt%Tb doped rutile TiO2 28788 433 0.5wt%Tm doped rutile TiO2 9770 392 1.0wt%Tm doped rutile TiO2 15735 6811 1.5wt%Tm doped rutile TiO2 1702 397 2.0wt%Tm doped rutile TiO2 29585 378 0.5wt%Dy doped rutile TiO2 14560 438 1.0wt%Dy doped rutile TiO2 9641 386 1.5wt%Dy doped rutile TiO2 16524 562 2.0wt%Dy doped rutile TiO2 14625 394 0.5wt%Er doped rutile TiO2 16815 9149 1.0wt%Er doped rutile TiO2 14388 405 1.5wt%Er doped rutile TiO2 16307 423 2.0wt%Er doped rutile TiO2 17204 643 0.5wt%Tm doped rutile TiO2 9770 392 1.0wt%Tm doped rutile TiO2 15735 6811 1.5wt%Tm doped rutile TiO2 17024 397 2.0wt%Tm doped rutile TiO2 29585 378 0.5wt%Y doped rutile TiO2 4049 337

sample average volume median size(nm)

average number median size(nm)

1.0wt%Y doped rutile TiO2 4153 342 1.5wt%Y doped rutile TiO2 3705 294 2.0wt%Y doped rutile TiO2 3950 312

From what has been discussed above, we can draw a qualitative conclusion, the crystal grain sizes are influenced by doping ratio. Most of RE elements, such as La, Ce, Nd, Sm, Eu, Dy and Er, doped TiO2 have the minimum particle size, when doping ratio is 1.0wt%. Therefore, the specific surface area of 1.0wt% RE doped rutile TiO2 are the biggest. It can be found that the particle size of La doped rutile TiO2 is smaller than any others. Therefore, 1.0wt% La doped rutile TiO2 is expected to be good at photocatalytic activity.

C. Zeta-potential analysis

Figure 3. Zeta potential of rutile TiO2 and RE (La3+, Ce4+, Pr3+, Nd3+, Sm3+ and Eu3+) doped rutile TiO2 (doping ratios are 0.5wt%, 1.0wt%, 1.5wt% and

2.0wt%) with different PH value

Figure 4. Zeta potential of rutile TiO2 and RE (Gd3+, Tb3+, Dy3+, Er3+, Tm3+ and Y3+) doped rutile TiO2 (doping ratios are 0.5wt%, 1.0wt%, 1.5wt% and

2.0wt%) with different PH value

Samples are scattered into the water, Zeta potential with different PH value of different doping radio RE-rutile TiO2 have been got, as shown in the Fig. 3 and Fig. 4. The isoelectric point of pure rutile TiO2 is 6.0. The isoelectric point of 0.5wt%, 1.0wt%, 1.5wt% and 2.0wt% La doped rutile TiO2

are all 6.5. This is consistent with the fact, that the isoelectric point of La3+ is 8.7~9.7. In conclusion, La doped rutile TiO2 can increase Zeta potential of rutile TiO2. The principle of Tm and Y are similar. The isoelectric point of 0.5wt%, 1.0wt%, 1.5wt% and 2.0wt% Ce doped rutile TiO2 are 4.0, 6.2, 5.2 and 3.9. The isoelectric point of CeO2 is 6.75. It is observed that 1.0wt% Ce doped rutile TiO2 got the best effect. The principle of Dy, Gd and Tb are similar. The isoelectric point of 0.5wt% Pr doped rutile TiO2 is 10.8. The isoelectric point of 1.0wt% Pr doped rutile TiO2 are 3.8, 7.4 and 8.1. The isoelectric point of 1.5wt% Pr doped rutile TiO2 are 3.7, 7.3 and 10.7. The isoelectric point of 2.0wt% Pr doped rutile TiO2 are 4.7, 7.4, and 8.1. The isoelectric point of Pr doped rutile TiO2 is more than one. The complex electronic structure of Pr caused rutile TiO2 produce more crystal defect. The principle of Sm, Er and Eu are similar. There are not any isoelectric points in the Zeta potential of Nd doped rutile. This illustrates Crystalline state of Nd doped rutile TiO2 is well. All RE doped rutile TiO2 except Nd makes Surface electrical of rutile TiO2 more complex. The reason for this maybe is doping RE make rutile TiO2 product more crystal imperfection.

D. Fluorescence analysis

Figure 5. Molecule fluorescence spectroscopy of rutile TiO2 and RE (La3+, Ce4+, Pr3+, Nd3+, Sm3+ and Eu3+, Gd3+, Tb3+, Dy3+, Er3+, Tm3+ and Y3+) doped

rutile TiO2 (doping ratios are 0.5wt%, 1.0wt%, 1.5wt% and 2.0wt%)

Molecular fluorescence spectroscopy is Secondary emission spectrum. The molecules absorbing energy jump from ground state to excitation state. Molecules, in the lowest vibrational level, jump to different levels of ground state. The energy emitted in the form of light. That is the molecular fluorescence.

Molecular fluorescence spectroscopy has nothing to do with wavelength of incoming ray, only depend on the level structure of the lowest vibration energy level and different levels of ground state of molecular first excited electronic state. Because the molecular fluorescence radiation takes the lowest vibration energy level of first excited electronic state as the early energy level always, molecular fluorescence spectrum directly reflects the information of molecular structure.

It is generally believe that FS spectrum derived from the recombination of photoproduction electron hole pair. Its abscissa (absorptive wavelength) correspond energy level transition. Its intensity closely related to the sample of the surface properties. The defect and impurity of the surfaces entered the electrons and holes for it to provide a new surface texture. They all will change their movement, lifetime and transition energy. Therefore, from Fig.5, we arrive at the following conclusions:

• We can find that there is not any characteristic peak of RE. This is consistent with the conclusion of XRD, that RE element not exist on the surface of the power but in crystal lattice.

• As shown in Fig.5, under 290nm light wave, the FS spectrum figure of Ce (Er, Gd, Tb and Tm) doped rutile TiO2 with different addition quantity. The characteristic peaks of 420nm and 490nm corresponding to the transition between Ti3d and O2p orbits. Surface absence of oxygen and defect of TiO2 caused the change of abscissa and peak intensity. Trapped electron and Ti-O-Tm bonding cause this. For one thing, surface defect state and impurity state acted as trap captured photoelectron, changed the separative efficiency of photon-generated carrier. For another thing, Ti-O-Tm bonding lowers the concentrations of surface absence of oxygen and defect, and then change the absorptive wavelength and intensity of fluorescence spectrum.

• According to the change of absorptive wavelength, the fluorescence spectrum of RE doped rutile TiO2 can be classified into two categories, that is redshift and blueshift. We can found that the maximal wavelength of 1.0wt%Ce, 2.0wt%Ce, 1.0wt%Er, 1.5wt%Er, 2.0wt%Er, Gd, Tb and Tm doped rutile TiO2 shows bathochromic effect. It corresponds to the rise of photoelectron and the decline of Ti-O-Tm bonding. The maximal wavelength of 0.5wt% Ce, 1.5wt%Ce and 0.5wt%Er doped rutile TiO2 shows blueshift effect. It corresponds to the decline of photoelectron and the rise of Ti-O-Tm bonding.

In sum, the results of fluorescence spectrum are used to prove RE element exist in crystal lattice. In addition, different RE elements and RE elements with Different doping had all strong impact on the FS spectrum of rutile TiO2.

IV. CONCLUSIONS This text comprehensively and systematically study RE

oxides (La3+, Ce4+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Er3+, Tm3+ and Y3+) doped rutile TiO2. The samples are synthesized at a low temperature. The power has been proved is rutile phase TiO2 by XRD. In addition, the particle size of RE doped rutile TiO2 is micron order through the particle size analysis.

TiO2 have consistently been studied by XRD, infrared spectrum (IR), Scanning Electron Microscope (SEM) and others. However, few people analyze the sample of TiO2 by zeta potential and FS spectrum. The results of zeta potential of

RE doped rutile TiO2 explain RE how to change the photoelectric activity by increase the lattice defect. The results of FS shown that RE how to influence on the electron transition and the electron trap of rutile TiO2

Such achievements have resulted in a good significance for guiding both the research on photoelectric properties of RE doping rutile TiO2 and the researchers in their selection of doping elements and doping ratio.

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