Decrease in the photoactivity of TiO2 pigment on doping with transition metals

J. Photochem.

Photobiol.

A: Chem.,

63 (1992)

367-375

Decrease in the photoactivity doping with transition metals Zhenghao Applied

Luot

Chernbtry

(Received

and Qing-Hua Department,

of TiOz pigment

on

Gaott

Shanghai Jiao Tong University, Shanghai 200030

April 22, 1991; accepted

367

(China)

September 12, 1991)

Abstract The decrease in the photoactivity of TiOp pigment (polycrystalline rutile powder) on doping with transition metals was investigated. Polycrystalline TiOz thin films doped with various elements to determine the best dopants were prepared by thermal decomposition of dopant nitrates on pure titanium substrates and their photoactivities were measured using surface photovoltage (SPV) spectra. Molybdenum and vanadium reduced the photoactivity. Ti02 powder (rutile) doped with molybdenum, vanadium or binary dopants was prepared by hydrolysis of TiCI.+ Compared with commercial samples, the photoactivity of TiO, powder doped with 1.5 mol.% MO, 1 mol.% V, 0.1 mol.% V plus 1 mol.% Al or 0.1 mol.% V plus 1 mol.% Pb was decreased on the basis of both SPV spectra and photochemical measurements. It may be that the d electrons of molybdenum (46) and vanadium (3d), as majority carriers in TiOa, can effectively quench the high energy photogenerated holes at the impurity levels introduced by doping within the band gap of TiOz.

1. Introduction Ti02, the best white pigment, is an n-type semiconductor which, on illumination, causes the so-called “chalking” of coatings: TiOz photo-oxidizes the polymeric binders within coatings, the pigment particles separate out and, finally, the continuous coatings become easily erasable powder [ 11. TiOa pigment, especially anatase, exhibits significant photodegradation without treatment. Chalking of TiOz is a photocatalytic oxidation process which occurs when the high energy photogenerated holes oxidize the smface hydroxyl ions of TiOz, [Ti4’-OH-], forming free radicals

Electron spin resonance (ESR) [2, 31 h as demonstrated the presence of these highly reactive radicals which easily break polymeric chains, degrade the polymer and result in chalking. Therefore, chalking can be reduced or prevented if the photocatalytic activity of TiOa is decreased. There are two methods which can be used to achieve

this:

tAuthor to whom correspondence should be addressed at: Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712-1062, USA. ftPresent address: Hawaii Natural Energy Institute, University of Hawaii at Manoa, Hondulu, HI 96822, USA.

lOlO-6030/92/$5.00

Q 1992 - Elsevier Sequoia. All rights reserved

368 (1) Quenching of -OH. TiOz particles can be coated with layer(s) of indifferent oxides such as Alz03, SiOz or ZnO to ensure a fast recombination of the majority of the free radicals [2, 41 2-OH

-

Hz0 + 1/202

(2) Annihilation of photogenerated holes [S]. The photogenerated electron-hole pairs can be annihilated by recombination process before they can diffuse into the vicinity of the [Ti4+ -OH-] species and react with them. In this study, we have attempted deep impurity levels which will enhance the indirect to dope Ti02 to ‘
details

Preparation of doped polycrystalline TiUz thin films The dopants were prepared as nitrate solutions (0.01 g cm-2). The dopant solution was dropped on a pure titanium substrate (1 cm X 1 cm) whose surface was previously cleaned with pure ketone, 10% HF, 10% oxalic acid, and rinsed with redistilled water. The doped thin films were then formed by thermal oxidation in air at 660 “C for 3 h. The thin films were allowed to cool naturally in the furnace and the SPV spectra of the samples were then measured to choose the best dopant elements. 2.1.

Preparation of doped polycrystaIline Ti02 (t-utile)powder An aqueous solution of TiCl, was hydrolysed and the screened dopant solution was stoichiometrically added. The dried precipitate was placed in a self-made Sic furnace and calcined in air at 1000 “C for 1 h. The powder was ground when cool. 2.2.

SPV measurement The apparatus for the measurement of SPV spectra is similar to a photoacoustic spectrometer. The light source was a 500 W xenon lamp and monochromatic light was obtained using a Jobin-Yvon HL high intensity monochromator. The reverse side (titanium substrate) of a thin film sample was polished until pure titanium was revealed. The sample was then sandwiched between two pieces of conducting glass with the doped side facing the incident light (see Fig. 1). The interfaces between the sample and the conducting glass were moistened with a drop of redistilled water. TiO, powder specimens were directly sandwiched between conducting glass without preliminary treatment. The photovoltage ZU. incident light wavelength was scanned and recorded. .The photoactivity of TiOz was determined by the SPV strength, i.e. its peak value. 2.3.

369

conducting

Fig.

1. Schematic

TABLE SPV

diagram

of the apparatus

glass

used for the measurement

of the SPV spectrum.

1

of TiO,

thin films doped

Dopant

SPV peak

Btank Bi CP

6400 4950 200 (A=400 228 (A=465 63 (A=390 47 (A=467 1.3

Fe” MO Pb V

673 3.9

“Photoresponse

with various

intensity

nm) nm) nm) nm)

(pV)

elements

(0.01

g cm-‘)

Dopant

SPV peak intensity

Al co CU”

2275 1106 - 140 30.5 35 18 713 628 580 1656

Mn” Ni Sr Zn

(A =371 (A = 465 (A=387 (A=467 (A=367 (A=384

(pV)

nm) nm) nm) nm) nm) nm)

broadened.

2.4. Photochemical measurement TiOa (1.5 g) and isopropyl alcohol (8 ml) were placed in a photoreactor (425 mmx 60 mm; quartz window, += 20 mm). The system was purged with oxygen and illuminated continuously with a 200 W mercury lamp under still oxygen for 8 h. The suspension was then centrifuged and the supernatant liquid was separated out to measure the ketone content by gas chromatography.

3. Results

and discussion

3.1. TiO, thin films The SPV measurements reveal that molybdenum and vapadium are the most efficient dopant elements; the peak values of the photovoltage of the doped thin films are reduced to about l/2000 of the blank sample (Table 1). Table 2 shows that a decrease in dopant concentration caused an increase in the SPV strength of molybdenumdoped samples, whereas no change was observed in vanadium-doped samples; this indicates that a small quantity of vanadium can effectively decrease the photoactivity of TiO*. In the molybdenum-doped thin films, the sublimation of Moo3 at about 650 “C may exert a strong impact on the relationship between molybdenum concentration and SPV. In our experiment the optical absorption edge shifts towards the red in vanadium-doped samples, A% > 415 nm. This drop in band gap width may be caused by the formation of continuous shallow impurity levels in the TiOa forbidden band

370

TABLE

2

SPV of MO-doped and V-doped Ti02 unit, 0.005 g cm-* as l/2 unit, etc.)

thin films at various concentrations

(0.01 g crr~-~ as 1

Dopant

SPV (CLV)

Dopant

SPV @V)

Mo MO MO MO MO

1.3 3.9 7.7 29.5 23.4

V V V V V

3.9 3.3 2.7 2.0 3.0

(1) (l/2) (l/4) (l/S> (l/16)

TABLE

(1) (l/2) (l/4) (l/8) (l/16)

3

SPV of binary-doped TiO, thin films (total Dopant Mo+Al MotPb

(l/S+ (l/8+

Mo+Zn MotMn MotBi

(l/8+1/16) (l/8+ 1/16Jb (l/8+ l/16)

“Photoresponse bPhotoresponse

l/16) l/16)

OAO1 g cm-*

as 1 unit)

SPV &V)

Dopant

1.2 6.5

V+AI V+Pb

(l/16+1/16) (l/16+1/16)

1.6 1.1

2.2 6.6 0.3 (A=368 1.1 (A=401

V+Zn V+Mo

(l/16+1/16) (l/16+1/8)

1.8 2.2

SPV (PV)

nm) nm)

narrowed. not broadened.

SPV of 99% Reagent Ti02 as 100 units

1

0

MO Concentration

2 (mole

%)

Fig. 2. SPV of TiOz ZJS.molybdenum concentration. after vanadium doping. Table 3 shows that binary doping with molybdenum is also effective. 3.2.

or vanadium

TiOz powder

Figures 2 and 3 show that TiO;! (rutile) powder samples are much less active on illumination when doped with molybdenum and vanadium. The relationships between SPV strength and dopant concentration in powder samples are very similar to those observed in thin films; the vanadium concentration was very high in the thin film

371

I[

SPV of 99% Reagent TiO2 as 100 units

J

0.2

0.4

0.6

V Concentration Fig. 3* SPV of TiO,

TABLE

0.x

(mole

1.0

1.2

%)

‘us. vanadium concentration.

4

SPV of TiOa powder doped with V +Af and V+

Pb (arbitrary unit, SPV of 99% reagent TiO, as 100 units)

Dopant (mol.%)

SPV

Dopant (mol.%)

SPV

O.lV +0.5A1 O.lV + l.OA1 0.05V + O.OSPb

2.04 1.94 21.49

O.lV+O.ZPb 0.1V-+0.4Pb O.lV+ l.OPb

8.72 5.19 1.97

samples, but not so high in the powder samples and, as a result, the optical absorption edge does not shift in the powder samples. Binary doping decreases the photoactivity of vanadium-doped TiOz powder (Table 4) but not molybdenum-doped TiOz powder. Comparisons of the photoactivities of the doped Ti02 powder samples with commercial samples are listed in Table 5 based on SPV measurement. These results are also confirmed by the photochemical method (Table 6). The results show explicitly that TiO, (rutile) powder samples doped with 1.5 mol.% MO, 1 mol.% V, 0.1 mol.% V plus 1 mol.% Al or 0.1 mol.% V plus 1 mol.% Pb have a reduced photoactivity. 3.3. Mechanism Electron spectroscopy for chemical analysis (ESCA) spectra of molybdenum-doped TiO;! powder show the existence of molybdenum(VI). Its X-ray diffraction pattern exhibits a rutile structure, but shows smaller peak values and a much more complicated line structure compared with that of pure rutile. However, the characteristic diffraction peaks of the dopant element are not observed. This implies that the doped molybdenum, in the form of molybdenum(VI), has substituted for titanium(IV) in the lattice, forming a solid solution. This is supported by the thermodynamic and structural data given in Table 7. When molybdenum substitutes for titanium in the rutile lattice, the two surplus 4d electrons will become the majority carriers of Ti02. ESCA shows a reduced binding decreases the positive charge on the energy of Ols, indicating that the substitution oxygen atom and results in an increase in the charge density on titanium since the covalent bond of Ti-0 is stronger than that of Mo-0. Hence, the two surplus d electrons are likely to be the recombination centres of the photogenerated holes when exposed to light and cause a decrease in SPV. On the other hand, Mo6+ may compete

372 TABLE

5

SPV of doped TiOz powder samples and commercial samples (arbitrary unit; A, anatase; R, rutile; C, coated; D, doped; SM, self-made)

TiO, sample

Treatment

SPV

reagent (P.R. China, A) Spectrum reagent (P.R. China, A)

None None

BlOl@ B102@ A-100” KA-IO” R820@

C (AW3) C G4W3) 7

100 72.25 70.72 17.53 70.72 13.51 4.62 - 2.72 14.12 -8.97 32.57 1.69 1.93 1.94 1.97

99%

(P.R. China, A) (P.R. China, A) (Japan, A) (Japan, A) (Japan, R)

CR800@ (U.S.A,

? C (A1,03,

R)

C (Al&) None D (1.5% D (1.0% D (0.1% D (0.1%

SM-1 (R) SM-2 (R) SM-3 (R) SM-4 (R) SM-5 (R)

TABLE

SiO,, ZnO,)

MO) V) V+l.O% Al) Vt 1.0% Pb)

(A= 323 (A = 368 (A= 322 (A=371

nm) nm) nm) nm)

6

Yield of acetone produced by the photo-oxidation of isopropyl alcohol in the presence of TiOz powder (arbitrary unit) TiOz sample

Ketone yield

TiO,

99% reagent R820@ Blank

100 7.8 10.4

SM-1 SM-2 SM-4 SM-5

TABLE

sample

Ketone yield 57.5 4.8 5.8 4.9

7

Comparisons of thermodynamic and structural properties ion radius (pm) [lo]

AG lMOKof oxide (kcal mol-‘) [II]

Bond geometry of oxide

Ti4+: 68 Mo6+: 62 V4+: 63 v5+: 59

Ti02: -255 Mo03: -224 VOz: - 206 v205: -454

Rutile octahedral Distorted octahedral Distorted octahedral Very distorted octahedral

for photogenerated MO”

+ e -

The existence via the route

electrons

with Ti4+

Mo5+ of Mo5+ has been dernonstrated

by ESR

[223. MO’+ may relax its energy

373

MO’+ ---+

Mo6+ +e

The net result of such a cycle is the annihilation of a photogenerated electron-hole pair. ESCA spectra of vanadium-doped sampfes show increases in the binding energies of TiZp and Q, and a decrease in the binding energy of VzPXR, implying that doped vanadium exists as VO, and a solid solution (Ti, -,V,O,) is probably formed (see Table 7). ESR studies also indicate that V4’ (3d’) substitutes for lattice titanium(W) after doping monocrystalline rutile with vanadium [13]. Although the 3d electron of vanadium is not as active as the 4d electron of molybdenum, it is reactive enough to trap a high energy photogenerated hole. Similarly, there is also a competing reaction of V4’ with Ti4+ for the capture of photogenerated electrons. It has been reported by Heller et al. [14] that the photoactivity of TQ pigments can be reduced by enriching the surface electron density to increase the surface recombination rate. In a similar manner, doping with molybdenum or vanadium introduces d electrons as donors to quench the photogenerated holes. Since the direct recombination rate of rutile is very small, the recombination process is inevitably achieved by the impurity levels in the TiO, forbidden band introduced by doping. Studies on the binding energies of transition metal impurities in TiOz using ESR and photoconductivity measurements [15] have shown that vanadium (no data for molybdenum are available) has two deep levels in the forbidden band which are the recombination centres for quenching the photogenerated holes, and therefore the photoactivity is reduced. The photoacoustic spectra (Fig. 4) show that only the doped samples exhibit strong photoacoustic signals located beyond the optical absorption edge. The photoacoustic spectra are indicative of an energy relaxation process of the absorbed optical energy through the release of heat. The strong photoacoustic signals in the longwavelength region can only be explained by electron transitions between energy levels smaller than the band gap of Ti02; this means that doping with molybdenum or vanadium introduces recombination centres in the TiOz forbidden band and enhances the surface recombination of the photogenerated electrons and holes.

PAS Signal

Wavelength (nm) Fig. 4. Photoacoustic spectra of self-made doped and commercial pure; 2, R820; 3, SM-1, 4, SM-2; 5, SM-4; 6, SM-5.

TiO, powder: 1, chemically

374 TABLE

8

Effects of applied water on the SPV measurement of a typical doped TiQ sample (4, lock-in amplifier phase angle; 7, lock-in amplifier time constant; A,, wavelength at SPV peak; A%, wavelength at optical absorption edge; R, resistance between the illuminated side and the dark side of the sample)

Dry Wet

3.4.

SPV

4 (deg)

7 (s)

A,,,

13.57 1.21

54 318

10 3

374 372

(nm)

AE, (nm)

R (Ma)

Noise

411 405

>20 4

Large Small

SPV measurement The use of redistilled

water to moisten the samples in the SPV experiments gives a good capacitive contact between the rigid surface of the TiO+ thin films and the conducting glass. Although the signals can easily be obtained without water for the powder samples due to a better surface contact, the experiment is much easier to perform and the noise is reduced when water is applied. Table 8 lists some interesting results from our experiment. After moistening the SPV strength, 4, 7, hEe, R and noise all decrease, while A,, shows no appreciable change. Although it is not clear why moistening cuts down the resistance between terminals, it can cause a decrease in SPV. Another reason for the decrease in SPV may be the absorption of incident light by water, giving rise to a variation in phase angle as well as a shift in the cut-off wavelength. Since A,,,= reflects the nature of the materials, it is not affected by the addition of water. 4. Conclusions

Chalking may be reduced by doping TiOz powder with certain transition metals. Doping with 1.5 mol.% MO, 1 mol% V, 0.1 mol.% V plus 1 mol.% Al or 0.1 mol% V plus 1 mol.% Pb causes a significant decrease in the photoactivity of TiO2 (rutile) powder. It is suggested that the doped molybdenum(W) or vanadium(W) substitutes for the lattice titanium(W), the 4d electron (molybdenum) or 3d electron (vanadium), acting as donor, effectively annihilates the high energy photogenerated holes, and the photoactivity of the TiOz pigment is reduced. The recombination process is achieved by the impurity levels in the forbidden band of the Ti02 polycrystal introduced by doping. The SPV spectrum is a sensitive and convenient surface analysis technique useful for the evaluation of photoactive materials, such as TiOz, and has extensive potential. Acknowledgment We thank China’s National

Natural

Science Foundation

for support of this work.

References 1 H. G. Vrllz, G. K$mpf and A. Klaeren, Farbe+Lack, 82 (1976) 805. 2 H. G. Viilz, G. Klimpf, H. G. Fitzky and A. Klaeren, in S. P. Pappas and F. H. Winslow (eds.), Photodegradation and Photostabilization of Coatings, ACS Symposium Series 151, American Chemical Society, Washington, DC, 1981, p. 163.

375

3 S. P. Pappas and R. M. Fisher, J. Paint TechnoL, 46 (1974) 65. 4 C. G. Roffey, Photopolymerization of Surface Coatings, Wiley, Chichester, 1982. 5 A. Heller, Begins Int. Conf: on Photochemistry, Beijing, Beijing Institute of Photographic Chemistry, Chinese Academy of Sciences, 1985, p. 479. 6 G. Irick, J. Appl. PO&~. Sci., 16 (1972) 2387. 7 A. M. Goodman, J. Appl. Phys., 32 (1961) 2550. 8 H. C. Gatos, J. Lagowski and R. Banisch, Phologr. Sci. Eng., 26 (1982) 42. 9 J. Lagowski and H. C. Gatos, J. Appl. Phys., 49 (1978) 2821. 10 R. C. Weast (ed.), CRC ffandbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1980, p. F-216. 11 I. Barin and 0. Knacke, Thermochemical Propriies of Znorgnnic Substances, Springer, Berlin, 1973. 12 T. Chang, Bull. Am. Phys. Sot., 88 (1963) 464. 13 H. J. Gerritsen and H. R. Lewis, Phys. Rev., 119 (1960) 1010. 14 A. Keller, Y. Degani, D. W. Johnson, Jr. and P. K. Gallagher, J. Phyx Chem., 91 (1987) 5987.

15 J. B. Goodenough Landok-Btimstein

Group

III, Vol.

and A. Numerical

17, Subvol.

Hamnett, Data

in 0.

Madelung,

and Functional

g., Springer,

Berlin,

M. Schulz

Relationships

1984, p. 430.

and

in Science

H.

Weiss

and

(eds.),

Technology,