Langmuir 1990,6, 206-209
206
rado). M.G. thanks the Swiss Office FBd6ral de 1'Energie (OFEN) for financial support. We gratefully acknowledge the assistance of Drs. Humphry-Baker and G. Rothenberger with the light-scattering experiments and their evaluation, as well as the skillful performance of the cyclic
voltammetry of P. Comte. We thank Dr. J. Moser for preparing the colloidal semiconductor solutions. Registry No. NaI,7681-82-5;MV2+,1910-42-5;Na+CF,SO;, 2926-30-9; oxygen, 7782-44-7.
ESR, XRD, and DRS Studies of Paramagnetic Ti3+ Ions in a Colloidal Solid of Titanium Oxide Prepared by the Hydrolysis of TiC13 Akira Ookubo Tomita Pharmaceutical Co., Ltd., Maruyama 85-1, Seto, Naruto, Tokushima 771-03, Japan
Eiji Kanezaki' Department of Chemical Science and Technology, Faculty of Engineering, University of Tokushima, Minamijosanjima 2-1, Tokushima 770, Japan
Kenta Ooi Government Industrial Research Institute, Shikoku, Hananomiya 2-3-3, Takamatsu 761, Japan Received January 30, 1989. In Final Form: June 15, 1989 By the hydrolysis of TiCl, in aqueous solution with urea, several differently blue colored solids of colloidal titanium oxide have been synthesized, colloidal solids which show commonly a broad absorption in the visible to the near-infrared wavelength region by means of diffuse reflection spectroscopy (DRS). The intensity of this band is proportional to that of the integrated Ti3+ESR signal, which indicates the origin of the blue coloration to be unoxidized Ti3+ ions in the solids. With the aid of X-ray diffraction (XRD), it is clear that Ti3+ ions exist in the Magn6li phases. Stability of Ti3+ ions is discussed.
Introduction Recently, there has been an increasing number of studies of low valence titanium ions in the solid state with the hope of developing new materials such as mixed valence compounds.l-' Trivalent (Ti3+) ions are easily obtainable, in particular, and have such a characteristic 3d electron in each cation that various spectroscopic studies have been ~ndertaken.'-~In these studies, Ti3+ions have been introduced by reducing Ti4+ ions at high temperature with h y d r ~ g e n l or - ~catalytically with Pt adatoms3 or by photoreducing them under UV irradiati~n'.'.~and so Ti3+ ions, however, locate only at the surface of TiO, solid, as many authors have indicated in these studies, and thus are easily oxidized to Ti4+ ions by oxygen in air. In order to avoid the oxidation, an approach has 011.~9'
(1) Gravelle, P. C.; Juillet, F.; Meriaudeau, P.; Teichner, S. J. Dis-
cuss. Faraday SOC.1971,52, 140.
(2) Iyengar, R. D.; Codell, M. Adu. Colloid Interface Sei. 1972, 3, 365. (3) Huizinga, T.; Prins, R. J. Phys. Chem. 1981, 85, 2156. (4) Iwamoto, N.; Hidaka, H.; Makino, Y. J. Non-Cryst. Solids 1983, 58, 131. (5) Howe, R. F.; Grltzel, M. J. Phys. Chem. 1985,89, 4495. (6) Pizzini, S.;Narducci, D.; Daverio, D.; Mari, C. M.; Morazzoni, F.; Gervasini, A. J. Chem. SOC.,Faraday Trans. 1 1987,83, 705. (7) Anpo, M.; Shima, T.; Fujii, T.; Suzuki, S.; Che,M. Chem. Lett. 1987,1997.
(8) Rajopadhye, N. R.; Bhoraskar, S. V.; Badrinarayan, S.; Sinha, A. P. B. J. Mater. Sei. 1988,23, 2631.
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been made whereby Ti3+ ions are dispersed in various kinds of inorganic In this attempt, Ti3+ ions are protected by surrounding glass matrixes against contact with oxygen but at the sacrifice of the possibility of examining surface activities in such an admixed system of heterovalent titanium ions. Although it has been noticed that a blue coloration accompanies the presence of Ti3+ ions, few authors have examined the quantitative relation between them.' A method has been proposed in which a Ti3+salt is hydrolyzed slowly with the presence of urea in an aqueous ~ o l u t i o n .With ~ this method, a blue titanium oxide is obtained, and the degree of the coloration has been controlled by changing the period of refluxing the solution in the synthetic procedure. In this oxide, an ESR resonance due to Ti3+ ions has been d e t e ~ t e d . ~ In this paper, as the first step in the entire understanding of low valenced titanium in the TiO, matrix, spectroscopic studies consisting of electron spin resonance (ESR), X-ray diffraction (XRD), and diffuse reflectance spectroscopy (DRS) are reported for several colloidal solid samples which have been obtained by the method above. In particular, the correlation between the spin density of Ti3+ions and the absorbance of the d-d transition is established. With these observations, it is concluded that (9) Ookubo, A,; Ooi, K.; Tomita, T. J. Mater. Sei., in press.
0 1990 American Chemical Society
ESR, XRD, and DRS Studies of Tis'
SampleA
'
'
Sample0
Langmuir, Vol. 6, No. 1. 1990 207
-
I5
35
25
45
Figure 1. TEM images of samples A and D. Particles I and I t see text.
281degree Figure 2. XRD patterns of samples A and D. Peaks b and r and vertical lines are assigned as the diffraction by brookite, rutile, and double oxides, respectively.
the origin of the blue color is due to Ti3+ ions which exist stably in the TiO, matrix as double oxide microcrystals. Furthermore, it is proved that the Ti3+ content in the matrix can be controlled by changing the period of refluxing of the reacting solution.
condition (pH 0.7),sample D was obtained under a more weakly acidic one (pH 2.5). This pH change is due to the neutralization by ammonia, which is produced in the hydrolysis of urea
2CQnm
ZCQnm
Experimental Section An aqueous solution of TiCI, and urea was refluxed with the molar ratio 1.00:2.62 in air with vigorous stirring. The pH value of thesolution was measured periodically. Every time &r refluxing for 2, 5, 6, 7, and 8 h, precipitate was separated from the solution by centrifugation, filtered under suction, washed with distilled water, and dried at 338 K. Samples A, B, C, D. and E denote the fine powders obtained in time order. Transmission electron microscopy (TEM) was carried out by using a Hitachi H-800electron microscope at 100 kV. A JEOL JDX-SP diffractometer with the Ni-filtered Cu K a line was used for X-ray diffracton measurements. All diffraction peaks were assigned by using the ASTM standard powder diffraction data. ESR spectra were measured by a computer-assisted JEOL JES-RE3X spectrometer with an X-band above 123 K. M d ulation frequency and field were 100 kHz and 2 C , respectively. Integrated spin numbers and exact g-values were determined by taking CuS0,.5H20 and a Mn2* salt as references, respectively. A Shimadzu UV-390 spectrophotometer was used for measuring diffuse reflection spectra of the powdered samples. BaSO, powder was used as the white standard. Infrared (IR) spectra were obtained by using a Hitachi 260-30 spectrophotometer with the KBr method. Results a n d Discussion Figure 1 shows TEM images of samples A and D. Sample A consists of disklike particles with a diameter of about 150 nm (particle I in Figure 1). In sample D, however, new particles below8 nm in size (particle I1 in Figure 1) are recognized within and on the crystals which are observed in sample A. The degree of crystal growth in sample D is less than that of sample A. XRD patterns of these two samples are illustrated in Figure 2. In sample A, several prominent peaks are assigned to the diffraction due to brookite," prominent peaks which are accompanied by weak peaks due to rutile." In the pattern of sample D, however, no prominent peaks are observed, which indicates the low crystallinity of sample D. The low crystallinity may be derived from the low regularity in atomic rearrangement on deposition. While sample A was obtained under an acidic
+ n,o
-
ZNH, + co, (1) It is s u p p e d that the small degree of regularity is related to the increased variety of coordinating fragments (NH,, CO,, etc.) to titanium on deposition. In the pattern of sample D, new weak peaks are noticed in Figure 2. As listed in Table I, these new peaks are assigned as the diffraction due to two double oxides, Ti OIlL3and Ti7013.14In both of these double oxides TiSP and Ti'+
CO(NH,),
ions coexist in the same lattice." The crystallite size calculated with Schemer's equation" is 7.7 nm from the halfwidth of Ti,O,, (03i) peak and agrees with the size estimated from the TEM Observation before. Both double oxides are also recognized in XRD patterns of the other blue samples (B, C, and E). ESR spectra of samples B-E both show a resonance with no hyperfine structure. The g-value is 1.96, which agrees with that of the paramagnetic Ti3+ions reported by Fairhunt et d.16 A plot of the logarithm of signal intensity vs the inverse of temperature exhibits a distinct knee point at 148 K as the temperature is increased. This knee point has been observed by some authors at 147 K, owing to the phase transition in Ti,O,, single crystal^.^^^^' Thus, this observation supports the presence of Ti60,, microcrystals in samples B-E. Another resonance due to 0,-superoxide anion radicals has been 2.00.1g20 This resoreported in TiO, solids with g nance is weakly observed in the spectrum of sample A although it may be hidden behind the intense Ti3+ resonance in the spectra of samples B-E. Figure 3 shows diffuse reflection spectra for samples B-E. An intense absorption which rises near 380 nm is due to the hand-gap transition in TiO, solid. On the long-wavelength side, a weak absorption rises gradually
-
(13) ASTM index No. 18.1401. (14) ASTM index No. 18.1403. (15) Klug. H. P.;Alexander, L. E.X-ray Diffraction Procedure for Polycrystalline and Amorphous Moteriob: Wiley: New York, 1956,p
491. ~.~~ (16) Fairhurst, S.A,; In& A. D.: Le Page,Y.;Morton, J. R.; R,.;PIP ston, K. F. Chem. Phys. Lett. 1983.95.444. l9.33,47,6. (17) Le Page,Y.:Strobel, P.J . Solid Stnte Chem. l9.33,47,6. (18) Nsccnche, C.; Meriaudeau. P.;Che, M.; Tench, A. J. Tmns. Forodav Soe. 1971,67,506. Faraday 1971.67.506. (19) Ragsi. J. N&e (19)Nature 1987,325,703. (20) Amorelli, A.: Evans. J. C.: Rmlands, C. C. J. Chen Chem.1. soe.. Soe.. Faraday Trons.I 1980.84, 1723.
208 Langmuir, Vol. 6, No.
I, 1990
Ookubo et al.
Table I. Assignment of X-ray Diffraction Peaks of Sample D d, A
a
4.96
40
106
3.89
90
120 i22,023 122 026
3.37 3.24 3.11 2.971 2.837
100 40 90 100 40
022 206 204 104 126,123
2.688 2.626 2.590 2.512 2.488
60 40 90 90 90
202,008 226
2.386 2.333
90 20
024 217 017,2i1 03i 1,1,ii
2.254 2.226 2.211 2.164 2.153
60
Reference 13. * Reference 14.
t
hkl
1110
104
108
sample D"
Ti,O,,b
Ti,O,," hkl
103 100 104
60 60
I/IO
121 121 105,122
4.71 3.88 3.78 3.36 3.30 3.14 3.01
40 20 90 100 40 90 100
024 021 203 203 102 123 202 025 224 022 215
2.751 2.655 2.627 2.574 2.508 2.486 2.404 2.373 2.327 2.287 2.218
20 60 40
014,2io 03i 117
2.207 2.167 2.138
120
40 90
d, A
60
90 90 40 40 40 40 60 60 60
40
dobw
A
U/IO)oba
7.96* 7.25* 5.05
30* 30' 30
3.77 3.35 3.24 3.12 2.93 2.81 2.75 2.68 2.62 2.55
60 40 50 30 50 30 30 20 20 40
2.48
100
2.37
20
2.23
30
2.18 2.13
90 30
* indicates an unknown peak.
r-----.
B
I
OLC
-
'0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 f (Rd')
Figure 4. Relation between the spin density and f(Rd'). Rd': see text. Figure 3. Diffuse reflection spectra of samples B-E. Rd: relative diffuse reflectance.
in the spectra of all samples, although the accurate location of the maximum and the presence of any associated bands are not determinable. This absorption is not observed in the reflection spectra of white Ti02 solids (rutile and anatase) and is assigned to the Ti3+ originated d-d transition which has been observed in the visible region of aqueous solution2' and in the visible to the near-infrared region of silicate glass.4 In this experiment, the absorbance a t 780 nm is tentatively assigned as representative of the d-d transition intensity, which is unavoidable because of the experimental condition. Samples B-E are blue, and the ordering in the degree of their coloration agrees with that of the absorbance due to this band. Figure 4 exhibits the relation between the spin density of Ti3+ions in ESR spectra and f(Rd'); f(Rd') = (1 - Rd'I2/2Rd' in the Kubelka-Munk equation, and Rd' is the reflectance relative to that of rutile at 780 nm. The spin density increases linearly with the increase in the magnitude of f(Rd'). This linearity was found to be repro(21) Pecsok, R.L.; Fletcher, A. N. Inorg. Chem. 1962, 1, 155.
ducible. The observation in Figure 3 confirms that the spin density in the oxide is linearly dependent upon the absorbance due to the d-d transition. A t this stage, it is safe to conclude that the origin of the blue color is the unoxidized Ti3+ ions that have been introduced into TiO, solid. This result indicates clearly that the spin density can be determined quantitatively with the aid of DRS. In a previous paper,g it was concluded that, in the condition below pH 1,Ti4+ions deposit as Ti02.nH20whereas Ti3+ ions deposit in the condition of pH 2-3. As is mentioned before, sample A was prepared with the precipitate which deposited at pH 0.7. Therefore, it is reasonable that no Ti3+ ions are detected in this sample. On the other hand, samples B-D were prepared from individual precipitates which deposit commonly a t pH 2-3. In this pH condition, Ti3+ ions in the reacting solution codeposit with Ti4+ ions and give mainly double oxides in solid. In the solid phase, Ti3+ ions in double oxides are stable, but the rest of the cations which exist substitutionally in the solid matrix are easily oxidized to Ti4+ ions (vide infra). If the deposition rate of Ti3+ ions from the solution is larger than the rate of Ti3+ consumption by the oxidation in the solid phase, the Ti3+ content in the solid phase increases with increasing reaction time,
Langmuir 1990,6, 209-217
209
which is the case in the preparation of the precipitates for samples B-D. A t the same time, the degree of coloration in sample E is smaller than that in sample D. This is accounted for by the fact that Ti3+ ions in solution are exhausted after 7 h of refluxing. This depletion is confirmed by measuring the amount of deposition periodically. Namely, only the oxidation of the Ti3+ ions in the solid phase occurs after sampling the precipitate for sample D. The stability of Ti3+ ions in the reacting solution may be aided by coordination of urea to Ti3+ ions. In order to determine which atom in urea coordinates with various metal ions, IR spectra have been used so far.22 In the IR spectrum of the dried mixture of urea and the TiC1, aqueous solution, u(C=O) is observed a t 1630 cm-', which is red shifted 53 cm-' from the location in the case of free urea molecules. This red shift indicates that the oxygen is the donor atom to Ti3+ ions. Thus, due both to the coordinative and the reductive nature of urea, many Ti3+ions remain in the solution until the end of deposition. Double oxide formation makes Ti3+ ions stable in the TiO, matrix through the stoichiometric bonding forma-
tion, a double oxide in which the valence for both kinds of titanium ion is satisfied fully. In the absence of double oxide, oxidation of Ti3+ ions is not suppressed. An excess 3d electron in each Ti3+ ion has the tendency to transfer itself, to diffuse toward a surface of a colloidal particle, and finally to be consumed there through the oxidation by dissolved oxygen in solution, for example. The instability of Ti3+ ions has been observed by Ragai and Sing when a suspension of amorphous titanium(II1) oxide was bubbled with oxygen23and by Howe and Gratzel when these ions are produced by means of the bandgap photoexcitation of TiO, in colloidal ~ o l u t i o n . ~ It is expected that the coexistence of such low valenced titanium ions with different electronic structure from that of the predominant Ti4+ ions may affect not only the bulk but also the surface properties. Study is now in progress on this aspect. In conclusion, the origin of the blue coloration is the Ti3+ ions. This species is stabilized in solid by the formation of double oxides, Ti6Ol1 and Ti,O,,.
(22) Penland, R. B.; Mizushima, S.; Curran, C.; Quagliano, J. V. J. Am. Chem. SOC.1957, 79,1575.
(23) Ragai, J.; Sing, K. S. W. J. Colloid Interface Sci. 1984, 101, 369.
Registry No. Ti3+,22541-75-9;TiO,, 13463-67-7;TiCl,, 770507-9; Ti,O,,, 12143-56-5; Ti,O,,, 12143-58-7;urea, 57-13-6.
Multiple Internal Reflection Fourier Transform Infrared Spectroscopic Studies of Thiocyanate Adsorption on Silver and Gold Diane B. Parry and Joel M. Harris* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Kevin Ashley* Department of Chemistry, San Jose State University, San Jose, California 95192 Received January 18, 1989. I n Final Form:'July 13, 1989 Conducting silver and gold coatings on silicon attenuated total reflectance (ATR) plates have been employed as transparent electrodes to monitor in situ surface electrochemistry. The multiple internal reflection Fourier transform infrared spectroscopy (MIRFTIRS) technique, used previously to study redox reactions on platinum and iron films, is applied in this work to the study of adsorption processes in the double-layer region, in particular, the adsorption of thiocyanate on silver and gold. The MIRFTIRS spectra were found to be essentially free of solution band interference and are compared to surface IR spectra of the same system obtained by other methods. Spectra of thiocyanate adsorbed on gold and silver surfaces have been recorded as a function of applied potential and thiocyanate concentration. Evidence of thiocyanate species absorbed to gold via both nitrogen and sulfur atoms has been obtained, while only S-bound thiocyanate was clearly observed on silver. Introduction
The spectroelectrochemical technique, multiple internal reflection Fourier transform infrared spectroscopy (MIRFTIRS),has previously been employed in redox studies involving iron and platinum films.'-3 The sensitiv(1) Neugebauer, H.; Neckel, A.; Nauer, G.;Brinda-Konopik,N.; Garnier, F.; Tourillon, G. J . Phys. (Les Ulis, Fr.) 1983, 44, 12. (2) Neugebauer, H.; Nauer, G.; Brinda-Konopik, N.; Kellner, R. Fresenius 2. Anal. Chem. 1983,314, 266.
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ity and surface selectivity of MIRFTIRS, however, suggest that it has much broader applications, both in terms of the chemical systems which can be studied and the combinations of metal films and ATR substrates available. The method is applied here to study adsorption of thiocyanate on silver and gold films within limits of applied potential where no Faradaic processes occur. (3) Pham, M.-C.; Adami, F.; Lacaze, P.-C.; Doucet, J.-P.; Dubois, J.-E. J.Electroanal. Chem. 1986,201, 413.
0 1990 American Chemical Society
