Characterization on the microstructures and optical performances of TiO2 doped with transition metals

Liu Xiuhua a, Deng Yi b, Zhang Yuchuan c, Zhou Yin-hang d

China Academy of Engineering Physics, Mianyang, Sichuan, China

aliuxiuhuajulia@163.com, bmayid0719@yahoo.com.cn, c1023922592@qq.com, dzhoubank@163.com

*Corresponding author. Tel.:+86 2484263; fax:+86 2484200. E-mail address:liuxiuhuajulia@163.com

Keywords: doping, TiO2, optical performances, transition metal

Abstract. The structures and optical performances of TiO2 doped with 4th

periodic transition metal

ions were investigated in this paper. The characterization results of X-ray photoelectron

spectroscopy and X-ray diffraction showed that the transition metal ions existed in oxidative states,

and composites formed because of the reaction between doped metal ions and TiO2. The absorption

spectroscopy of TiO2 doped with zinc was mainly in ultraviolet region, close to that of the pure

TiO2. While for TiO2 doped with other transition metal ions including V, Cr, Mn, Fe, Co, Ni and Cu

ions, the absorption spectroscopies covered ultraviolet region and visible light region, much broader

than that of the pure TiO2.

Introduction

As a kind of clean and efficient photocatalyst, TiO2 has an attractive application prospect in

many fields. But traditional TiO2 can only absorb ultraviolet light (λ< 387 nm) and solar energy

utilization rate is very low (below 5%). Expanding the TiO2 absorption spectrum to the visible light

region is one of the key technologies to improve the utilization rate of solar energy.

H.B. Song [1] studied the N and Co doped TiO2 catalysts. Results showed that mesoporous

N/Co-TiO2 photocatalyst exhibited the highest photocatalytic activity. K. Kikoin[2] studied the

ferromagnetism in non-stoichiometric TiO2 doped with transition metals, and construct the theory of

ferromagnetism in magnetically doped oxygen deficient anatase TiO2. Y.L. Shang[3] studied the

TiO2 doped with different metal oxides, and the results showed that doping Na2O, ZrO2, Al2O3 or

CeO2 could enhance the electrorheological performance of the TiO2 material, whereas, doping CaO

or ZnO would decrease the ER activity of the material. Relative researches focused on the changes

of the catalytic activity, ferromagnetism and electrorheological property after doping the transition

metal ions, while there have been little research on the changes of the microstructure and other

performance of TiO2 before and after the doping of transition metal ions. The microstructures and

optical absorption performances of TiO2 catalyst doped of 4th

periodic transition metal ions were

studied in this paper, hoping to lay the foundation for the development of visible light catalyst and

catalyst modification mechanism.

Experiment

2.1 Main reagent

Butyl titanate (C.P.); Anhydrous ethanol (A.R.); Hydrochloric acid; NH4VO3 (A.R.); K2Cr2O7

(A.R.); MnCl2·4H2O (A.R.); FeCl3·6H2O (A.R.); Co(NO3) 2·6H2O (A.R.); NiCl2·6H2O (A.R.);

CuCl2·6H2O (A.R.); ZnCl2 (A.R.); Distilled water

Advanced Materials Research Vols. 887-888 (2014) pp 388-394Online available since 2014/Feb/06 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.887-888.388

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-14/11/14,17:04:02)

2.2 Fabrication of TM/TiO2 powders

Buff transparent solution A was prepared by dripping 10 ml Ti(OBu)4(c.p.) into certain amount

of ethanol(A.R.) and stirring. Solution B was prepared by mixing certain amount of ethanol,

hydrochloric acid transition metal compound solution. The transition metal-doped TiO2 collosol

formed after dripping solution B into A under extreme stir.The collosol was parched under an

infrared lamp, rubbed into powder, and calcined for 2 hrs at set temperature in a muffle furnace.

Thus the transition metal-doped TiO2 powder (shortened form: TM/TiO2) was obtained.

2.3 Characterization of TM/TiO2 powders

A PE Lambda 35 ultraviolet-visible spectrophotometer equipped with an integrating sphere

accessory were used for recording the ultraviolet - visible diffuse reflectance absorption spectra of

the TM/TiO2 powder. X-ray photoelectron spectroscopy (XPS) of the TM/TiO2 powders were

analyzed by using an XSAM800 photoelectron spectrometer (Kratos Company). The fitting of XPS

curves was done by curve fitting software. X-ray diffraction (XRD) patterns of the TM/TiO2

catalyst were determined using a D8 Advance X-ray Diffraction Instrument (BRUKER Company).

The BET surface area of the catalysts was measured by a dynamic nitrogen adsorption method at 77

K using a ST-2000 BET determinator (Beijing Analysis Instrument Plant).

Results and discussion

3.1 Component analysis of TM/TiO2 powders

Fig.1 is the Ti2p(a) and O1s(b) XPS spectra of TM/TiO2 powder, which is close to the XPS

spectra of the pure TiO2.The binding energies of the Ti2p3/2 peaks are located at 458.4～458.7 eV

symmetrically, indicating that the titanium atoms are in +4 oxidation state. The O1s peaks are more

complex. There are crystal lattice oxygen of TiO2, surface hydroxide oxygen and the crystal lattice

oxygen of other transition metal oxide. The Ti2p peak value for TM/TiO2 powder shifts to higher

binding energy comparing with that for the pure TiO2 powder. This chemical shift might be caused

by different chemical environment of atoms in the molecules and can be explained by the principle

of electronegativity. The chemical shifts are basically caused by the changes of potential energy

from the valence electron transfer, and the valence electron transfer is closely related to the

corresponding element electronegativity. Under the intense nucleus coulomb effect, the inner

electrons in the atoms have certain electronic binding energy. At the same time, the inner electrons

get a shielding effect by outer electrons. When the valence electrons transfer to the atoms with

larger electronegativity, the electron density decreases of the atoms with smaller electronegativity,

thus the shielding effect decreases and the electronic binding energy increases. The increase of the

binding energy of Ti2p peak for TM/TiO2 powders is due to the formation of Ti-O-TM bond on the

contact interface between doping elements and the titanium dioxide. The electronegativities of

doped transition metal (in table 1) are bigger than the electronegativity of titanium (1.54). In

Ti-O-TM bond, the electron cloud move to transition metal atom, leading to less electron for the

outer orbit of titanium atom. Therefore, the electronic shield action of outer orbit reduces and the

binding energy of Ti2p increases.

470 465 460 455 450

Mn/TiO2

Cr/TiO2

V/TiO2

Fe/TiO2

pure TiO2

Co/TiO2

Cu/TiO2

Ni/TiO2

Zn/TiO2

Binding Engery(eV)

538 536 534 532 530 528 526 524 522 520

Zn/TiO2

Cu/TiO2

Ni/TiO2

V/TiO2

pure TiO2

Cr/TiO2

Fe/TiO2

Mn/TiO2

Co/TiO2

Binding Engery(eV)

Fig.1 XPS spectra of Ti2p(a) and O1s(b)for TM/TiO2 powder

(a) (b)

Advanced Materials Research Vols. 887-888 389

Due to different elements, the location and shape of XPS spectra for transition elements in

TM/TiO2 powder differ greatly. The 2p XPS spectra of V, Cr, Mn, Ni and Cu in TM/TiO2 powders

are shown in fig.2. There are three XPS peaks in the fitted V2p XPS spectra of V/TiO2 powder. One

is the partner peak of O1s, which overlaps with peak of V2p1/2. The peaks locate at 517.0 eV are the

characteristic peaks of V2p3/2 for V2O5, indicating that vanadium exists in V2O5. For the Cr2p XPS

spectra of Cr/TiO2 powder, the peaks locate at 578.2 eV and 587.8 eV are the characteristic peaks of

Cr2p3/2 and Cr2p1/2 for CrO3 respectively, indicating that chromium exists in CrO3. For the Mn2p

XPS spectra of Mn/TiO2 powder, the peaks locate at 642.6 eV and 654.2 eV are the characteristic

peaks of Mn2p3/2 and Mn2p1/2 for MnO2 respectively, indicating that manganese exists in MnO2.

525 520 515 510

Binging Engery(eV)

595 590 585 580 575 570 565

Binging Engery(eV)

665 660 655 650 645 640 635

Binging Engery(eV)

890 880 870 860 850 840

Binding Energy(eV)

960 940

Binging Engery(eV)

Fig.2 XPS spectra of doped elements for the surface of TM/TiO2 powder

(a):V; (b):Cr; (c):Mn; (d):Ni; (e):Cu

For the Ni2p XPS spectra of Ni/TiO2 powder, the peaks locate at 855.5eV and 873.9eV are the

characteristic peaks of Ni2p3/2 and Ni2p1/2. The normal binding energy for the Ni2p3/2 of NiO is

853.3eV. The gap is 2.2 eV, indicating that nickel is not exists in NiO. The Ni/TiO2 powders were

furtherly characterized by XRD method(The XRD pattern of Ni/TiO2 powders is omitted). Obvious

characteristic peaks of NiTiO3 appear in XRD pattern, consistent with standard pattern of NiTiO3,

implying that NiTiO3 crystal has grown completely in Ni/TiO2 powder calcined at 973K. NiTiO3

belongs to hexagonal system，having similar close packing structure to rutile TiO2. The former is

cubic close packing, and the latter is distorted hexagonal close packing.

For the Cu2p XPS spectra of Cu/TiO2 powder, the peaks locate at 933.0 eV and 952.9 eV are

the characteristic peaks of Cu2p3/2 and Cu2p1/2 for CuO respectively, indicating that copper exists in

CuO. The XRD pattern of the Cu/TiO2 catalyst calcined at 973K(The XRD pattern of Cu/TiO2

powder is omitted) also shows that copper exists in CuO. For the XPS spectra of Fe/TiO2, Co/TiO2

and Zn/TiO2 powders have been analyzed in references [4-6]. Iron exists mainly in Fe2O3, and it

forms a titanium−iron solid solution with titania. Cobalt and zinc exist in CoTiO3 and ZnTiO3

respectively.

Table 1. Electronegativity，ion radii of transitional metal and the modalities in TiO2 powder

Atom Ti V Cr Mn Fe Co Ni Cu Zn

Electronegativity 1.54 1.63 1.66 1.55 1.83 1.88 1.91 1.90 1.65

Existing form

— V2O5 CrO3 MnO2 Fe2O3 CoTiO3 NiTiO3 CuO ZnTiO3

radii of 6-coordinate

ions, pm 74.5 68 58 67 69 79 83.0 87 88.0

(c) (b) (a)

(d) (e)

390 Advances in Materials and Materials Processing IV

The crystal lattice constants, axial ratios(c/a) and cell volumes(V) of TiO2 in the TM/TiO2

powders were obtained through Rietveld refining of XRD patterns by Bruker AXS Topas software.

The data are showed in table 2.

Table 2. Lattice parameters，crystal size and content of TiO2 in different structures in TM/TiO2

powders

powder Calcined

temperature, K

Structure of

TiO2 a, 0.1nm c, 0.1nm c/a

V,

0.001nm3 Size, nm Content，%

TiO2

773 Anatase 3.7843 9.5022 2.5109 136.08 13.4 100

973 Anatase 3.7836 9.5133 2.5145 136.19 107.2 2.07

973 Rutile 4.5928 2.9599 0.6445 62.44 111.8 97.93

V/TiO2 773 Anatase 3.7856 9.5098 2.5121 136.28 23.9 100

Cr/TiO2 773 Anatase 3.7858 9.5057 2.5109 136.24 35.8 100

Mn/TiO2

773 Anatase 3.7868 9.5047 2.5100 136.29 15.0 100

973 Anatase 3.7844 9.5116 2.5133 136.22 151.2 2.59

973 Rutile 4.5904 2.9583 0.6445 62.34 102.9 97.41

Fe/TiO2

773 Anatase 3.7862 9.5095 2.5116 136.32 19.4 100

973 Anatase 3.7838 9.5214 2.5164 136.32 115.1 84.79

973 Rutile 4.5949 2.9600 0.6442 62.49 189.7 15.21

Co/TiO2

773 Anatase 3.7862 9.5092 2.5115 136.31 20.5 100

973 Anatase 3.7836 9.5184 2.5157 136.26 91.5 4.47

973 Rutile 4.5927 2.9595 0.6444 62.42 182.7 95.53

Ni/TiO2

773 Anatase 3.7863 9.5057 2.5106 136.27 20.2 100

973 Anatase 3.7830 9.5231 2.5173 136.28 117.3 87.24

973 Rutile 4.5939 2.9601 0.6444 62.47 153.7 12.76

Cu/TiO2 773 Anatase 3.7815 9.4458 2.4979 135.07 15.8 100

973 Rutile 4.5935 2.9596 0.6443 62.45 118.3 100

Zn/TiO2 773 Anatase 3.7783 9.3752 2.4813 133.84 23.0 100

973 Rutile 4.5925 2.9594 0.6444 62.42 99.6 100

The doping of the transition metal ions have little effect on the crystal lattice constant of

anatase TiO2. The crystal lattice constant a decreases, and c increases after the TiO2 were doped

with nickel ions. While the crystal lattice constant a, c and cell volume all increase when calcined at

both 773K and 973 K after the doping of iron ions. The change tendency of the lattice constant and

the axial ratio differ with various metal ions doping. The anisotropic change of crystal lattice

constants reflects the microscopic action between doped metal ions and TiO2. The explaination of

this phenomenon need further theoretical calculation. The content of anatase TiO2 in the TM/TiO2

powders are showed in table 2. After doped with iron and nickel, the content of anatase TiO2 is

higher than that of the pure TiO2 powder calcined at the same temperature, implying that the doping

of iron and nickel ions inhibits the phase changing of titanium dioxide from anatase to rutile

structure. On the contrary, the doping of copper and zinc ions promoted the transformation of TiO2

from anatase to rutile structure.

3.2 Optical performance of TM/TiO2 powders

The UV-Vis absorption spectra of TM/TiO2 powders were characterized by a PE Lambda 35

ultraviolet-visible spectrophotometer. The UV-visible Diffuse Reflectance adsorption spectroscopies

are shown in Fig.4.

Advanced Materials Research Vols. 887-888 391

200 300 400 500 600 700 800

0.25

0.50

0.75

1.00

1.25

1.50

Abso

rban

ceWavelength,nm

V/TiO2

Cr/TiO2

Mn/TiO2

Fe/TiO2

Co/TiO2

Ni/TiO2

Cu/TiO2

Zn/TiO2

TiO2

Fig.4 UV-visible Diffuse Reflectance adsorption spectroscopy of TiO2 powder doped with transition

metal elements

In the ultraviolet range, TiO2 shows very strong spectral absorption, which is due to the

electron transition between the valence band and conduction band of TiO2 (from valence band to

conduction band). The two largest absorption near 233 nm and 344 nm is should be attributed to the

charge transfer from O2-

to Ti4+

[7]. Except Zn/TiO2 powder, other TM/TiO2 powders have obvious

absorption in the visible light range. The doping of V5+

, Cr6+

, Mn4+

, Fe3+

, Co2+

, Ni2+

and Cu2+

all

realized substantial redshift of absorption wavelength to visible light.

The approximate energy of band gap Eg for TM/TiO2 powders were calculated by using the

relationship between the light absorption coefficient, the photon energy and band gap energy (Eg).

The calculation method is in literature [5]. The light absorption threshold(λg) of TM/TiO2 powders

are calculated according to the formula 1240 /g gEλ = from the energy of band gap and the results are

shown in table 3.

Table 3 Energy of band gap of TM/TiO2 powder Doping metal Ti V Cr Mn Fe Co Ni Cu Zn

Eg,eV 2.78 1.53 1.81/1.55 <1.55 1.52 1.56 1.75 2.15 2.89

λg,nm 446 810 685/800 >800 816 795 707 577 429

The UV-Vis. absorption spectra of TM/TiO2 powders are significantly different to that of pure

TiO2 and corresponding metal oxides. The absorption wavelength of pure TiO2 powder is less than

446 nm. The absorption wavelength of pure V2O5 is less than 420 nm[8], while the absorption

wavelength of V/TiO2 is in 200 nm~ 800 nm, having obvious red shift compared with the pure TiO2

powder. Sarah Klosek’s study also show that the ultraviolet visible absorption spectrum of V-doped

TiO2 has obvious red shift comparing with the pure TiO2 [9]. The mixture of chromium oxide and

TiO2 has strong absorption below 425nm, weak absorption between 425~ 520 nm and no absorption

over 525 nm [10]. But the absorption band edge of Cr/TiO2 powder red shift to 685 nm, and a new

absorption band between 685 ~ 800 nm comes into being. MnO2 has light absorption within 200 ~

600 nm. The absorption coefficient augments along with the increase of absorption wavelength. The

absorption curve shows "slope". There is no absorption band edge, and the semiconductor quality is

very poor [11]. Mn/TiO2 absorption wavelength can be extended over 800 nm. The absorption

coefficient decreases along with the increase of absorption wavelength. The light absorption of

Mn/TiO2 powder between 300nm to 500 nm is due to the charge transfer from octahedral Mn4+

to

Ti4+

, and the absorption between 500 ~ 500 nm is considered to produce by the electron transition

from 6A1g→

4A1g and

6A1g→

4T2g in crystal field of octahedral Mn

4+ [12]. The forbidden bandwidth

for Fe2O3 is 2.2 eV [13], and the maximum absorption wavelength is 564 nm. The mixture of Fe2O3

and TiO2 has visible light absorption between 400~600 nm [8], the maximum absorption

wavelength of Fe/TiO2 powder expands to 816 nm. In the Co/TiO2 powder, cobalt exists in CoTiO3.

CoTiO3 can absorb the light in 400 ~ 750 nm and the strongest absorption is at 600 nm or so [14].

There is light absorption for Co/TiO2 powder under 795 nm, and the new absorption appears

392 Advances in Materials and Materials Processing IV

between 500nm to 795nm, M. Iwasaki [15] considered that CoTiO3 arouses the absorption between

500nm to 795nm.

In the Ni/TiO2 powder, nickel exists in NiTiO3. Ilmenite NiTiO3 have strong absorption

between 350~650 nm [14]. R.S. Singh reported that the band gap is 2.12 eV [16]. The light

absorption wavelength of Ni/TiO2 powder has broadened to 707nm, and the width of band gap

decreases. The light absorption of CuO is mainly centralized in the ultraviolet region, and have no

absorption over 400nm [17]. The light absorption wavelength of Cu/TiO2 powder has broaden to

816nm, and the strong absorption focus on 400~577nm. The light absorption performance of

Zn/TiO2 powder is close to that of pure TiO2 powder [18], has no absorption over 429 nm.

After being doped into the TiO2 sol, the metal ions have been turned into metal oxides or metal

composites through a series of processing. Because of the difference of existing form, the optical

performances of TM/TiO2 powders are significantly different from that of metal oxide and pure

TiO2. Even if the absorption spectra of metal oxide and pure TiO2 add together, that will not overlap

with absorption spectra of TM/TiO2 powders. The above phenomenon illuminates that the doping

metal ions is not simply mechanical mixing with TiO2, but react with each other. The analysis

results in section 2.1 also illustrate this point.

After impurity metal ions doped TiO2, many separated intermediate levels of impurities in the

forbidden band form. The intermediate levels of impurities not only can accept the excitated

electrons in valence band of TiO2, but also can absorb photons to make electronic transition to the

conduction band of TiO2. Completing both processes need to overcome less potential energy. The

metal oxide materials of vanadium, cadmium, manganese, iron, cobalt, nickel and copper all have

semiconductor properties. The valence band and conduction band positions are obvious different

from that of TiO2, leading to the change of excitation energy and the migration direction for the

photo induced electrons and holes. The electron orbits of doped atoms have a direct effect to

decrease the width of band gap of TiO2. This point is also testified by the theoretical calculation of

K.N. Song [19]. According to the crystal field theory, the 3d orbit of Ti atom is split into t2g state

and eg state. When doping with vanadium, the t2g state appear near the bottom of the conduction

band, and electrons can transit directly from the top of valence band to the intermediate levels of

impurities. When doping with cadmium, manganese and cobalt, the t2g states nearly appear in the

middle of the forbidden band, and electrons are likely to transit from the top of valence band to the

intermediate levels of impurities. When doping with iron, nickel and copper, the t2g states appear in

the forbidden band, close to the top of the valence band, and electrons are likely to transit from the

intermediate levels of impurities to the conduction band. The above three conditions all may make

the absorption band of TiO2 red shift. However, the state density of zinc is very close to that of TiO2,

so the doping of zinc has little effect on the optical performances of the TiO2.

Conclusions

The TiO2 doped with 4th

periodic transition metal ions were fabricated. The structures and

optical performances were investigated in this paper. Some metal ions doped in TiO2 form metal

oxide and some react with TiO2 to form composite. The transition metal doping introduce

intermediate bands into the narrowed forbidden gap of TiO2, thus affect the optical performances of

TiO2. The doping of V5+

, Cr6+

, Mn4+

, Fe3+

, Co2+

, Ni2+

and Cu2+

all increases the light absorption

threshold of TiO2, achieves the red shift of visible light absorption wavelength significantly.

However, the doping of zinc has little effect on the light absorption wavelength and the range of the

TiO2.

Advanced Materials Research Vols. 887-888 393

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Metals 10.4028/www.scientific.net/AMR.887-888.388

DOI References

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