Preparation and Characterization of Single-Modified TiO2 for

ARTICLE pubs.acs.org/IECR

Preparation and Characterization of Single-Modified TiO2 for Pigmentary Applications Barbara Grzmil,* Marta Glen, Bogumiz Kic, and Krzysztof Lubkowski West Pomeranian University of Technology, Szczecin, Institute of Chemical and Environment Engineering, Poland, 70-322 Szczecin, Puzaskiego 10 ABSTRACT: In the present work the influence of different additives, calculated to ZrO2, ZnO, and B2O3, on the anataserutile phase transformation was investigated. The chemical composition of prepared titania samples was determined by the selective leaching method, powder X-ray diffraction analysis, and FT-IR measurements. The effect of small amounts of additives on the optical properties, photoactivity, and surface area of rutile-phased TiO2 for pigmentary applications was examined using spectrophotometer, SEM, and BET measurements. ZrO2 acted as an inhibitor of the TiO2 phase transformation and the addition of ZnO or B2O3 to TiO2 accelerated rutile formation at 750 °C. ZrO2 addition in TiO2 could form separate ZrO2 phase and solid solution of Zr with Ti. Zinc partly reacted with titanium forming cophase TiZn2O4. Boron was located in TiO2 in the form of soluble compound, B2O3. Almost all optical properties were lower for modified TiO2 in comparison with the corresponding unmodified sample. The SBET increased with increase of ZrO2 in TiO2 and decreased with the increase of ZnO in TiO2. The photoactivity depended on the kind and contents of introduced additive and was lower for TiO2 modified with ZrO2 or ZnO than for TiO2B2O3.

’ INTRODUCTION Among inorganic pigments the most important and prevalent are titanium dioxide pigments known as titanium white. In the world trade there are about 400 kinds of titanium white which differ in production technology, crystalline form, properties, and area of interest.1 The use of titanium dioxide pigments is very wide. About 60% of world production is consumed by the dye and lacquer industry, 22% is used in the plastics industry, and 12% in paper industry. Nonpigmentary applications of titanium dioxide do not exceed 3% of the annual production.2 Titanium dioxide has three different polymorphic forms occurring in nature: rutile, anatase, and brookite, from which only anatase and rutile are technologically meaningful.3 Rutile is characterized by a higher density, higher packing density of atoms in its structure, higher hardness, and refractive index.4 Its pigmentary properties such as brightness, hiding power, tinting strength, opacity, and photostability are better than those of anatase.5 Anatase transforms exothermally into rutile. Inclusion of additives can be used to both elevate and depress the anatase rutile transformation.611 Doping TiO2 with ZrO2 resulted in inhibition of the anataserutile phase transformation due to the incorporation of Zr ions into the anatase lattice.12 It was found that the addition of ZnO to TiO2 can noticeably lower onset transformation temperature.13 TiO2 reacts with ZnO at high temperature forming zinc titanates.14 Boron doping may efficiently inhibit the grain growth and facilitate the anatase rutile phase transformation before the formation of diboron trioxide phase.15 It was inferred that boron was present in the form of B3þ in B-doped TiO2 samples and was likely to weave into the interstitial TiO2 structure. A layer of diboron trioxide phase on the surface of TiO2 nanoparticles suppresses diffusion between anatase particles in direct contact and limits their ability to act as surface nucleation sites for rutile. r 2011 American Chemical Society

Different TiO2 synthesis methods can bring about various particle sizes and thus contribute to the specific distribution of dopants within the particles and properties of the prepared material.16 Calcination in solgel method results in the coalescence of particles, which increases the size and decreases the surface area of the TiO2 samples.17 The nonhydrolytic solgel route is an alternative pathway for the preparation of metal oxides.1820 Comparing to the hydrolytic route, the nonhydrolytic method makes the morphologies and microstructures of obtained metal oxides more controllable. In pigmentary applications photocatalytic properties of TiO2 are undesirable. Oxidation of the organic substance is the reason for degradation of the materials which manifests itself with a decreased mechanical strength, embrittlement, and impairment of the appearance by losing the gloss, changes of color, or chalking.21 A photocatalytic activity of TiO2 (in the presence of UV light) depends upon the rate of electronhole recombination, number of electronsholes created, phase composition, surface area, adsorption properties for organic compounds, and crystallite size.22 In TiO2 with low photocatalytic activity, the rate of the surface charge carrier recombination process should be maximized, but the interfacial charge-transfer process should be minimized. The effective way to decrease the photoactivity of TiO2 is to defect its crystal lattice in order to generate electron traps for the holes formed as a result of radiation. This can be achieved by incorporating various additives into the TiO2 crystal lattice.2327 Titania modified with ZrO2 showed higher photocatalytic activity than pure TiO2, which was confirmed by different Received: July 28, 2010 Accepted: April 27, 2011 Revised: March 26, 2011 Published: April 27, 2011 6535

dx.doi.org/10.1021/ie1016078 | Ind. Eng. Chem. Res. 2011, 50, 6535–6542

Industrial & Engineering Chemistry Research reactions.2830 Yu and co-workers found that Ti1-xZrxO2 solid solutions exhibit higher photocatalytic activity than pure anatase for the degradation of acetone in air.31 It was shown that anatase doped with Zn had higher photoactivity (decomposition of phenol and methyl orange) compared with pure anatase.32,33 All B-doped TiO2 (120 mol % B) nanoparticles calcined at 500 °C showed higher photocatalytic activity than pure TiO2 sample in the photocatalytic reaction of nicotinamide adenine dinucleotide regeneration under UV light irradiation.15 It was also shown that photocatalytic activity of anatase for methylene blue decomposition increased with a growing amount of boron.34 The second group of methods which allow decreasing the catalytic activity of TiO2 is the one which slows down the radical formation on the pigment surface. Two methods can be mentioned here: removal of oxygen, water, and hydroxyl groups from the surface, and pigment microencapsulation.35 The latter consists in covering the pigment surface with a layer which prevents the electronhole pair from redox cycle on the pigment surface. In commercial practice, the surface of modified titanium dioxide is covered mainly with the following oxides: SiO2, Al2O3, CeO2, B2O3, and ZrO2, although tin, zinc, phosphorus oxides, and nitrates are also used.36,37 Despite a lot of work dedicated to photoactivity of titanium dioxide, most of it has been focused on the properties of photoactive anatase-phased TiO2 modified with additives in significant amounts. Very limited published works have been concerned with the properties of pigmentary rutile-phased TiO2 or with a commercial material.36,3841 In the present work the influence of various additives on the anataserutile phase transformation was investigated. The chemical composition of prepared titania samples was determined and the effect of single additives (zirconium, zinc, boron) on the optical properties of rutile-phased TiO2 was examined. The research was aimed at obtaining pigmentary rutile-phased TiO2 with low photoactivity, therefore small amounts of additives were introduced. As a starting material hydrated anatasephased titanium dioxide with rutile nuclei was used.

’ EXPERIMENTAL SECTION Sample Preparation. The starting material was technicalgrade hydrated titanium dioxide (HTD) as a concentrated suspension containing 37.4 wt % of TiO2 and 3.0 wt % of rutile nuclei. It was an indirect product from the industrial production of TiO2 with the sulfate method. Reagent-grade Zr(SO4)2, ZnSO4, and H3BO3 were dissolved in distilled water. Solutions of modifying agents (calculated to ZrO2, ZnO, B2O3) were introduced to HTD. Contents of ZrO2, ZnO, or B2O3 in TiO2 were changed respectively from 0.03 to 0.19 mol %, from 0.05 to 0.3 mol %, or from 0.06 to 0.34 mol %. The obtained pulp after thorough mixing (mechanical stirrer, 25 rpm, time 0.5 h) was transferred to an evaporating dish and then inserted into a laboratory muffle furnace (LM 312.13) heated to assumed temperature. The prepared samples, for investigations of the anataserutile phase transformation, modifiers’ distribution, optical properties, and photoactivity, were calcined for 1 h at 700, 750, and 800 °C for TiO2 modified with ZnO (0.050.3 mol %) or B2O3 (0.060.34 mol %), and for 1 h at 700, 750, 800, and 850 °C for TiO2 modified with ZrO2 (0.030.19 mol %). TiO2 samples modified with 27.8 mol % ZrO2, 2 mol % ZnO, and 4 and 6 mol % B2O3 for the purpose of X-ray diffraction analysis were calcined for 2 h at 1000 °C.

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Unmodified TiO2 was calcined at 700, 750, 800, and 850 °C. Additionally, pure B2O3 for FT-IR measurements was obtained at 1000 °C. Sample Characterization. The X-ray diffraction analysis (X’Pert PRO Philips diffractometer with Cu KR radiation) was used to determine the content of rutile and phase composition in the calcined titanium dioxide samples. The relative abundance of anatase and rutile phases was calculated from the (101) reflection of anatase and the (110) reflection of rutile. The content of rutile in the samples was determined from the following equation: W R ¼ 100=ð1 þ I A =k 3 I R Þ

ð1Þ

where WR is the content of rutile, IA and IR are the peak intensities of anatase (101) and rutile (110), and k is the coefficient (the ratio of peak intensity (101) 100 wt % of anatase to the peak intensity (110) 100 wt % of rutile). FT-IR measurements (FT/IR-430 Jasco spectrophotometer, wavenumber range 4000 to 400 cm1) were conducted for investigations of TiO2 phase composition. The surface morphologies of titania samples were observed and assessed by scanning electron microscopy operating at 20 kV (SEM, JEOL JSM-6100). BrunauerEmmettTeller surface area (SBET) measurements of selected TiO2 samples were also conducted using Micrometrics Quadrasorb SI Quantachrome Instrument. The analysis gas was nitrogen. N2 adsorption/desorption measurements were carried out at liquid N2 temperature. Distribution of Additives in Modified TiO2. Distribution of additives (Zr, Zn, B) in the investigated samples of TiO2 was determined with a selective leaching method.42 Demineralized water, solution of ammonia (0.5 mol 3 L1), sodium-EDTA (0.1 mol 3 L1), and hydrochloric acid (6 mol 3 L1) were used for leaching the additives and titanium from the calcined samples of TiO2. After calcination, TiO2 samples (2.5 g) were milled and mixed with an appropriate leaching solution. The prepared suspension was shaken in a water bath shaker at room temperature for 2 h. The solid phase was separated by centrifuging (High Speed Brushless Centrifuge MPW-350, 8000 rpm, 20 min). The contents of additives and titanium in the liquid phase were determined with the ICP-AES method (Optima 5300 DV, Perkin-Elmer). The leaching degree was defined as a ratio of the weight of the element leached into the liquid phase to the weight of that element contained in the titanium dioxide before leaching. Optical Properties and Photoactivity. The optical properties of unmodified and modified rutile TiO2 samples were characterized by measuring the color in the gray and white system. In the gray system the relative lightening power (tinctorial strength, TcS) and gray tone (spectral characteristics, SCx) and in the white system the reflectance, brightness, and white tone of TiO2 samples were determined. The gray paste consisted of reference or examined titanium dioxide, linseedtung oil, colloidal silica (Aerosil 200, Degussa AG), and carbon black with mass ratio of respective ingredients 1:0.84:0.05:0.03. The white paste was composed of the same components as the gray one but it did not contain carbon black. The mass ratio of TiO2 sample, linseed-tung oil, and colloidal silica in the white paste was 1:0.87:0.05. The procedure of the tests involved the following: sample milling in the laboratory mortar grinder, batching adequate amount of milled sample and paste either for measures in the gray or white system, mulling the sample with paste, applying the mulled paste on the test card and measuring the color using Konica Minolta CM-600d spectrophotometer 6536

dx.doi.org/10.1021/ie1016078 |Ind. Eng. Chem. Res. 2011, 50, 6535–6542

Industrial & Engineering Chemistry Research

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(Standard Illuminant C, 2° Standard Observer). The optical properties were calculated from X, Y, and Z values using CIE XYZ system. The relative lightening power (TcS) and gray tone (SCx) were calculated from the following formulas: TcSs ¼ TcSr þ ðY s Y r Þ 3 100

ð2Þ

SCx s ¼ SCx r þ ðZs X s Þ ðZr X r Þ

ð3Þ

where TcSs and SCxs are the relative lightening power and gray tone of the examined TiO2; TcSr and SCxr are known values of the relative lightening power and gray tone of the reference TiO2; Xs, Ys, and Zs are average value of trichromatic components X, Y, and Z of the examined TiO2; and Xr, Yr, and Zr are known average values of trichromatic components X, Y, and Z of the reference TiO2. The brightness and white tone were calculated from the formulas Bs ¼ Br þ ðY s Y r Þ

ð4Þ

WTs ¼ WTr þ ðZs X s Þ ðZr X r Þ

ð5Þ

where Bs and WTs are the brightness and white tone the examined TiO2, and Br and WTr are known values of the brightness and white tone of the reference TiO2. Photoactivity was characterized by the white leadglycerin test. The procedure of the test involved preparation of an aqueous paste containing a TiO2 sample, glycerin, colloidal silica, and basic lead carbonate with mass ratio of respective ingredients 1:1.40:0.08:0.02. The obtained paste was irradiated with an UV vis light for 1 h with a radiation intensity of 500 W/m2 (climatic chamber Atlas SUNTEST XLSþ, Xenon lamp 290800 nm, dose 1800 kJ/m2). During the exposure of samples to the UVvis radiation the photooxidation of glycerin followed by reduction of Pb2þ to Pb0 is proceeded. The discoloration of the paste induced by photoreaction was evaluated by measuring the ΔE* parameter using CIE L*a*b* system (Konica Minolta CM-600d spectrophotometer, Standard Illuminant C, 2° Standard Observer). ΔE* expresses the total chromatic change. The lower the ΔE*, the more photostable the pigment is. The color stability (photoactivity ΔE*) was calculated from the measurements of L*a*b* values of TiO2 samples before and after UVvis irradiation with the presented formula pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð6Þ ΔE ¼ ΔL2 þ Δa2 þ Δb2

Figure 1. Influence of ZrO2 addition and calcination temperature on the anataserutile phase transformation degree.

Figure 2. Influence of ZnO addition and calcination temperature on the anataserutile phase transformation degree.

where ΔL* is the lightness change of TiO2 sample after irradiation with x dose; Δa*, Δb* are the color change of TiO2 sample after irradiation with x dose.

’ RESULTS AND DISCUSSION AnataseRutile Phase Transformation. The anataserutile phase transformation of the calcined TiO2 samples was studied using powder XRD analysis. The influence of Zr(IV), Zn(II), or B(III) on the anataserutile phase transformation was investigated. Contents of ZrO2, ZnO, or B2O3 in TiO2 were changed respectively from 0.03 to 0.19 mol %, from 0.05 to 0.3 mol %, or from 0.06 to 0.34 mol %. It was found that at 750 and 800 °C, with an increase of ZrO2 additive, the fraction of rutile in calcination products gradually decreased with the increasing amount of this modifier in TiO2

Figure 3. Influence of B2O3 addition and calcination temperature on the anatase-rutile phase transformation degree.

(Figure 1). The anataserutile phase transformation degree changed from 64% to 28% at 750 °C and from 97% to 86% at 6537



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Table 1. Contents of Additives in Modified Titanium Dioxide content of additive type (mol %) 0

0.34 B2O3

0.29 ZnO

0.19 ZrO2

solvent

concentration

content of element in solution

content of Ti in solution

leaching degree of

leaching degree of

(mol 3 L1)

(mmol 3 L1)

(mmol 3 L1)

element (%)

Ti (%)

HCl

6

EDTA

0.1

NH4OH

0.5

H2O

0.08

0.01

0.1 2.03

0.02 96

HCl

6

2.03

EDTA

0.1

2.13

98

NH4OH H2O

0.5

2.16 0.06

100 3

HCl

6

0.91

EDTA

0.1

0.80 0.02

NH4OH

0.5

HCl

6

EDTA

0.1

NH4OH

0.5

800 °C. However, the rutilization degree at 700 and 850 °C did not depend on the amount of zirconium additive and it was about 68% and 100%, respectively. The influence of increasing content of zinc in titanium dioxide on the anataserutile phase transformation degree at temperature of 700 and 800 °C was not observed (Figure 2). The transformation degree was about 810% and 100%, respectively. However, an increase of zinc addition at 750 °C decreased transformation temperature. The degree of rutilization increased from 64% to 87%. The influence of boron additive on the anataserutile phase transformation was similar to the influence of zinc (Figure 3). A small increase of the rutilization degree from 64% to about 77% was observed with an increase of B2O3 in TiO2 at 750 °C. The influence of boron additive on the anataserutile phase transformation degree at calcination temperature of 700 and 800 °C was not observed. The transformation degree was about 710% and about 100%, respectively. In comparison to ZnO or B2O3 the addition of ZrO2 to TiO2 caused the increase of the rutilization temperature by 50 °C acting thus as an inhibitor of the TiO2 phase transformation. The addition of ZnO or B2O3 to TiO2 did not cause any change of the transformation temperature range but at 750 °C it accelerated rutile formation. Observed differences in the influence of ZnO, B2O3 or ZrO2 additives on the anataserutile phase transformation can result from different mechanisms of the reaction with titanium dioxide. Ionic radius of Ti4þ is 0.068 nm. Because of the larger ionic radius of Zr4þ (0.080 nm) this modifier can remain on the titanium dioxide surface and form a separate phase ZrO2. Formed ZrO2 may retard the anataserutile phase transformation. Zn2þ with ionic radius of 0.074 nm probably substitutes Ti4þ in the titanium dioxide crystal lattice. Ionic radius of boron(III) is 0.020 nm therefore small amounts of this modifier may locate in the interstitial sites of the titanium dioxide crystal lattice and facilitate the anataserutile phase transformation. Determination of Additives Distribution in TiO2. To determine the chemical composition of prepared titania samples, the distribution of the additives was studied with a selective leaching method. Characteristics of investigated unmodified and

0.12

0.15

96

49

0.02

0.02

43 1 0.12

0.02

Figure 4. X-ray diffraction patterns of titanium dioxide modified with ZrO2 calcined at 1000 °C; rutile (•), ZrO2 (r); (a) 4 mol % ZrO2; (b) 7.8 mol % ZrO2.

modified rutile TiO2 samples and the results obtained from the leaching of these materials with various solutions are compiled in Table 1. Leached samples included TiO2 modified with 0.19 mol % ZrO2, 0.29 mol % ZnO, or 0.34 mol % B2O3 and were calcined respectively at 850, 800, and 800 °C. Titanium dioxide modified with zirconium (calculated to 0.19 mol % of ZrO2) was leached. Solutions of hydrochloric acid, EDTA, and aqua ammonia were used. In obtained solutions even trace amounts of zirconium were not determined. Only trace amounts of titanium were released to the solution of hydrochloric acid. However the same amount of titanium was released to solution of hydrochloric acid during the leaching of unmodified titanium dioxide. This testifies that zirconium reacts with TiO2 and forms partly soluble phase of a Ti1-xZrxO2 type or zirconium does not form a compound with titanium and it occurs in modified TiO2 in the form of partly soluble, separate phase ZrO2. The XRD patterns were obtained for higher, calculated to ZrO2, contents (Figure 4). The objective was to verify the phase formation. It can be seen from Figure 4 that for 4 mol % ZrO2 a weak diffraction peak at 2θ = 30.3° was exhibited, indicating 6538



Figure 5. Relation between 2θ position and lattice plane distances d110 for the increasing ZrO2 contents in TiO2.

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Figure 7. X-ray diffraction patterns of titanium dioxide modified with ZnO calcined at 1000 °C before and after leaching with 6 mol 3 L1 HCl: rutile (•), TiZn2O4 (Δ).

Figure 6. X-ray diffraction patterns of modified titanium dioxide calcined at 1000 °C: rutile (•), TiZn2O4 (Δ); (a) 2 mol % ZnO; (b) 4.6 mol % B2O3.

Figure 8. FT-IR spectra of samples calcined at 1000 °C: (a) pure B2O3; (b) unmodified TiO2; (c) TiO2 modified with 4.6 mol % B2O3.

ZrO2 phase in the sample (JCPDS-01-079-1771). It is more noticeable for 7.8 mol % ZrO2 where new peaks of zirconium dioxide phase at 2θ = 50.4° and 2θ = 51.1° are apparent. The rest of the peaks correspond to the rutile phase (JCPDS-01-0724817). It was also observed that the increase of ZrO2 contents resulted in the 100% peak shift to smaller angles toward higher lattice plane distances (Figure 5). This shift informs about replacing Ti atoms with smaller ionic radius with Zr atoms with higher ionic radius, thus solid solution can be formed next to separate phase ZrO2. The formation of either separate ZrO2 or solid solution of Zr with Ti can restrict contact between grains and retard the anatase to rutile phase transformation. In our investigations the inhibiting influence of Zr(IV) on the anatase-rutile phase transformation was confirmed. It was suggested elsewhere that additives located in the intergranular spacers or on the surface of grains inhibit rutilization.43 Zinc contents after leaching of modified TiO2 (calculated to 0.29 mol % of ZnO) in solution of hydrochloric acid and EDTA was similar and amounted to 0.91 and 0.80 mmol 3 L1, respectively (Table 1). The degree of this element leaching was 49 and 43%, respectively. In EDTA solution no titanium was found,

whereas in solution of hydrochloric acid the content of titanium was 0.15 mmol 3 L1. This testifies that during the calcination process about 57% of zinc reacts with titanium and forms cophase TiZn2O4 which is slightly soluble in solution of hydrochloric acid. Hence 43% of zinc remains as ZnO. The formation of cophase with titanium and zinc, for 2 mol % ZnO in TiO2, is shown in Figure 6. Diffraction peaks at 2θ = 29.9, 35.2, 42.8, 53.1, 56.7, 62.1, and 73.7° are characteristic of TiZn2O4 (JCPDS-01077-0014). The evidence of solubility of this phase can be seen in Figure 7. The TiO2 sample modified with 2 mol % ZnO was also leached and subjected to XRD analysis. The decrease of TiZn2O4 peak intensity at 2θ = 35.2° was observed after leaching with HCl solution. The degree of boron leaching from modified TiO2 (calculated to 0.34 mol % of B2O3) amounted to about 100% for all the solutions used (Table 1). However, only trace amounts of titanium were transferred to the solution of hydrochloric acid. This proves that boron is located in TiO2 in the form of soluble compound, presumably B2O3. The XRD analysis could not confirm any formation of boron phase, probably because of its amorphous state (Figure 6). Therefore FT-IR measurements 6539



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Table 2. Optical Properties and Specific Surface Area of Unmodified and Modified TiO2 additive

contents (mol %)

brightness

white tone

relative lightening power

gray tone

SBET (m2/g)

0 (850 °C)

93.4

9.3

1380

1.6

6.1

0 (800 °C)

93.1

9.4

1360

0.4

7.5

0.06

91.0

9.6

1210

2.2

4.9

0.13

92.4

9.7

1250

1.9

5.9

0.19

91.7

9.5

1280

1.8

6.0

0.09

92.4

9.4

1220

1.4

8.0

0.19

92.4

9.6

1220

1.9

6.8

0.29 0.11

92.5 92.7

9.5 9.5

1220 1270

2.0 1.2

5.8 7.2

0.23

91.8

9.0

1260

1.3

8.8

0.34

92.2

9.2

1270

1.3

6.8

ZrO2 (850 °C)

ZnO (800 °C)

B2O3 (800 °C)

Figure 9. Influence of additives on the photoactivity ΔE* of TiO2.

were conducted additionally (Figure 8). The band at 1200 cm1 is assigned to the BO stretching vibrations of BO3 units44 and appeared in TiO2 modified with 4.6 mol % B2O3. Moreover another band has been observed at 2255 cm1. In unmodified titanium dioxide it does not exist. However, in comparison with obtained B2O3, an absorption at 2255 cm1also appears. That is to say TiO2 is modified with B2O3. Optical Properties and Photoactivity. Optical properties, photoactivity, specific surface area, and surface morphology were investigated for the samples with the rutilization degree of c.a. 100%, thus TiO2 modified with ZrO2 (calcined at 850 °C) and TiO2 modified with ZnO or B2O3 (calcined at 800 °C) were examined. The objective of the following experiments was to observe the effect of single additives on the color and photoactivity of modified TiO2. In the presented work also pure TiO2 calcined at 850 °C (TiO2ZrO2 series) and 800 °C (TiO2ZnO or TiO2- B2O3 series) was subjected to measurements of optical properties, photocatalytic activity, and specific surface area. The optical properties of unmodified and modified rutile were characterized by measuring the color in the white and gray system. In the white system the brightness and white tone, whereas in the gray system the relative lightening power and gray tone (Table 2) of TiO2 samples were determined. The specific surface area was also given in Table 2. In comparison with unmodified TiO2 calcined at 800 °C, unmodified TiO2 calcined at 850 °C had higher relative lightening

power but lower gray tone and specific surface area. The brightness and white tone were very similar for these two unmodified samples. Analyzing the color of TiO2 modified with ZrO2, ZnO, or B2O3, it was found that TiO2 with B2O3 had the highest white (from 9.5 to 9.2) and gray tone (from 1.2 to 1.3) of all modified samples. The white tone for TiO2ZrO2 and TiO2 ZnO was very similar and oscillated around value 9.6. The increasing contents of ZrO2 in TiO2 caused the increase of the gray tone (from 2.2 to 1.8) and the increasing contents of ZnO in TiO2 brought about the decrease of the gray tone (from 1.4 to 2.0). The change of the brightness with the increasing contents of any modifier was not observed. The relative lightening power was increasing with the increasing contents of ZrO2 in titanium dioxide (from 1210 to 1280) but no differences were observed for TiO2 modified with ZnO or B2O3. However the relative lightening power was higher for TiO2B2O3. In relation to the corresponding reference sample of unmodified titanium dioxide, calcined either at 850 or 800 °C, almost all optical properties were worse for modified TiO2. Only samples modified with B2O3 had higher white tone values than unmodified TiO2. The photocatalytic activity of TiO2 samples was characterized by the white leadglycerin test. The results are shown in Figure 9. Titanium dioxide modified with B2O3 had the highest photoactivity. The highest photostability was observed for TiO2ZrO2 series. The TiO2 photocatalytic activity decreased with the increase of 00.29 mol % ZnO (from 21.1 to 20.1) and 00.19 mol % ZrO2 (from 20.6 to 19.6) and increased with the increase of 00.34 mol % B2O3 (from 29.9 to 33.3). Although there is a relation between the photoactivity and content of modifier, a connection between ΔE* and SBET is incoherent. The specific surface area of investigated samples was very similar, however increased with the increase of ZrO2 in TiO2 and dereased with the increase of ZnO in titanium dioxide. The SBET of TiO2B2O3 series was the highest for 0.23 mol % B2O3. It was also observed that the surface morphology of all the examined TiO2 samples (for measurements of photoactivity and optical properties) was nearly the same and the shapes of the grains were irregular. Example SEM images of unmodified TiO2 (calcined at 800 and 850 °C) and TiO2 modified with 0.13 mol % ZrO2, 0.19 mol % ZnO, and 0.23 mol % B2O3 (calcined at 850, 800, and 800 °C, respectively) are shown in Figure 10 . It can be assumed from SBET measurements and SEM results that differences in photocatalytic activity and optical properties 6540



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Figure 10. SEM images of unmodified TiO2 calcined at 800 °C (A) or 850 °C (B) and TiO2 modified with 0.13 mol % ZrO2 (C), 0.19 mol % ZnO (D), and 0.23 mol % B2O3 (E), calcined respectively at 850, 800, and 800 °C.

of titanium dioxide depended on the kind and contents of introduced additives (in the examined range) but not on the TiO2 surface or its morphology.

’ CONCLUSIONS In the present work the influence of various additives on the anataserutile phase transformation was investigated. The chemical composition of prepared titania samples was determined and the effect of single additives (zirconium, zinc, boron) on the optical properties of rutile-phased TiO2 was examined. The research was aimed at obtaining pigmentary rutile-phased TiO2 with low photoactivity, therefore small amounts of additives were introduced. The results indicate that small amounts of ZrO2 in TiO2 caused the increase of the rutilization temperature by 50 °C in comparison to ZnO or B2O3 additives acting thus as an inhibitor of the TiO2 phase transformation. The addition of ZnO or B2O3 to TiO2 did not cause any change of the transformation temperature range but at 750 °C it accelerated rutile formation. It was proved that ZrO2 modifying of titanium dioxide could form separate ZrO2 phase and solid solution of Zr with Ti. Zinc partly reacted with titanium forming cophase TiZn2O4 and the rest of zinc remained as ZnO. The presence of boron located in TiO2 in the form of soluble compound, B2O3, was also confirmed. The effect of single additives on the color, photoactivity, specific surface area, and surface morphology of modified TiO2 was investigated. Unmodified TiO2, calcined at 850 °C, had higher relative lightening power but lower gray tone and specific surface area than unmodified TiO2 calcined at 800 °C. The brightness and the white tone were very similar of these two unmodified samples. It was found that TiO2 with B2O3 had the highest white and gray tone of all modified samples. The white tone for TiO2ZrO2 and TiO2ZnO series was very similar. The increasing contents of ZrO2 in TiO2 caused the increase of the gray tone whereas the increasing contents of ZnO in TiO2 brought about the decrease of the gray tone. The change of the brightness with the increasing contents of any modifier was not observed. The relative lightening power was increasing with the

increasing contents of ZrO2 in titanium dioxide but no differences were observed for TiO2 modified with ZnO or B2O3. The measured values of the photocatalytic activity were lower for TiO2 modified with ZrO2 or ZnO than for TiO2 modified with B2O3. The photostability and optical properties of titanium dioxide depended on the kind and contents of introduced additives.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: (48 91) 449 47 30. Fax: (48 91) 449 46 86. E-mail: barbara. [email protected].

’ ACKNOWLEDGMENT This work was funded from financial support on science for 20082011. ’ REFERENCES (1) Winkler, J. Titanium Dioxide; Vincentz Network: Hannover, 2003. (2) Da-browski, W.; Tymejczyk, A.; Lubkowska, A. Properties and Application of Titanium Dioxide Pigments; Chemical Plant “Police” S.A.: Police, 2006. (3) Hu, Y.; Tsai, H.-L.; Huang, C.-L. Phase transformation of precipitated TiO2 nanoparticles. Mater. Sci. Eng. 2003, A344, 209. (4) Chichina, M.; Tichy, M.; Churpita, O.; Hubicka, Z. A study of the barrier torch discharge applied for the thin film deposition. WDS ’05 Proceedings of Contributed Papers, Part II; MatfyzPress: Prague; 2005, p 325. (5) Lewis, P. A. Pigment Handbook. Properties and Economics; John Wiley & Sons: Canada, 1988. (6) Gesenhues, U. Calcination of metatitanic acid to titanium dioxide white pigments. Chem. Eng. Technol. 2001, 24, 7. (7) Al-Salim, N. I.; Bagshaw, S. A.; Bittar, A.; Kemmitt, T.; McQuillan, A. J.; Mills, A. M.; Ryan, M. J. Characterisation and activity of sol-gel-prepared TiO2 photocatalysts modified with Ca, Sr or Ba ion additives. J. Mater. Chem. 2000, 10, 2358. (8) Kim, J.; Song, K. C.; Foncillas, S.; Pratsinis, S. E. Dopants for synthesis of stable bimodally porous titania. J. Eur. Ceram. Soc. 2001, 21, 2863. 6541


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Preparation and Characterization of Single-Modified TiO2 for

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