1988
J . Phys. Chem. 1989, 93, 1988-1992
Electrical and Photoadsorptive Properties of Valence-Controlled a-FeOOH K. Kaneko,* N. Inoue, and T. Ishikawat Department of Chemistry, Faculty of Science, Chiba University. Yayoi, Chiba 260, Japan (Received: March I I , 1988; In Final Form: May 23, 1988)
Ti-, Cu-, and Al-added a-FeOOH crystals were prepared by coprecipitation of each foreign ion with Fe parent ions to obtain valence-controlled a-FeOOH. The electrical conductivity of the doped a-FeOOH was examined over the temperature range 303-383 K and frequency range of dc, 1 kHz to 10 MHz. The electrical conductivities of the Ti- and Cu-doped a-FeOOH increase markedly with the doping, while A1 addition does not affect the conductivity. The Ti- and Cu-doped a-FeOOH exhibit n-type semiconductivity, inferred from the conductivity drop upon the introduction of oxygen gas. The observed electrical conductivity changes can be interpreted by a new mechanism of valence control in hydroxides with defect chemical reactions. The effect of photoillumination on the SOz adsorptivity of Ti-doped a-FeOOH equilibrating with 33 kPa of SOz at 303 K was examined. The increment of SOz adsorption by photoillumination is related to changes in the electrical conductivity and morphological aspects of crystals due to doping.
Introduction
Nonstoichiometry frequently governs the surface chemical properties of transition-metal The nonstoichiometry of the metal oxides can be controlled by doping of foreign metal ions of valency different from the parent ion^.^-^ The foreign metal ions can be doped in the parent oxides by heating an intimate mixture of the parent and impurity oxides. In the case of oxygen-deficient oxides, the occupancy of a foreign metal ion of larger valency on the normal cation site creates both a quasi-free electron and an oxygen vacancy, which play important roles in the electrical conductivity and surface activity of the oxides. a-FezO3 is a representative oxygen-deficient oxide and exhibits n-type s e m i c o n d ~ c t i v i t y .The ~ ~ ~introduction of tetravalent ions such as Ti4+,Ge4+,and Sn4+enhances the n-type semiconductivity. The addition of divalent ions such as Cu2+,Mnz+,and Crz+gives n-type behavior, whereas Ca2+and Mg2+produce p-type behavior. Although the role of tetravalent ions in the mixed valence state of a-FezO3 is established, that of divalent ions is not clearly understood. a-FeOOH is known as a mineral (geoethite), a common constituent of surface water sediments, soils, and atmospheric rusts of iron-bases alloys and a magnetic tape precursor. a-FeOOH has a crystal structure close to that of a-Fez03;13914 a-FeOOH transforms into a-Fez03 through dehydration upon heating. Synthetic a-FeOOH is oxygen deficient, showing low electronic conductivity by the hopping of d electrons on Fe(I1) to Fe(II1) as well as a-Fez03.15-17Partial dehydration of surface hydroxyls increases markedly the electrical conductivity.'* The surface chemical properties of synthetic a-FeOOH have been studied from various aspects.'+z5 Knowing the relationship between the surface properties and the nonstoichiometry is essential in the elucidation of the role of a-FeOOH in natural environments; the preparation of valence-controlled a-FeOOH is expected. However, it is difficult to dope foreign metal ions in the a-FeOOH lattice by heating without transformation of a-FeOOH into a-Fe203. a-FeOOH has high chemisorption activity for SO2, and its electrical conductivity sensitively changes with SO, chemisorption.z6z8 A preceding letterz9reported that the SOz adsorptivity of a-FeOOH is enhanced by illumination with near-UV and visible light. The surface chemical properties of Cu-containing a-FeOOH was already reported.30 In this article, we prepared valencecontrolled a-FeOOH by the coprecipitation of Ti(1V) and Cu(I1) and examined their electrical conductivity and SOz photoadsorptivity. Experimental Section Preparation of Doped a-FeOOH. Ti-doped a-FeOOH (Ti/Fe: 1.2-5.6 at. %) was prepared by hydrolyzing a mixed solution of 'Present address: Osaka University of Education, Tennoji, Osaka 543, Japan.
0022-3654/89/2093-1988.$01.50/0
Fez(S04)3and Ti(S04)zsolutions at 303 K for 20 days. Cu- or Al-doped a-FeOOH was prepared from the mixed solution of Fe2(S04), and CuS04 or Alz(S0.J3 solutions in a similar way. The dopant content was colorimetrically analyzed. The Ti(1V) content was determined with 420-nm light after the addition of 3% H z 0 2solution to the HzSO4 solution of the Ti-doped crystals. The Cu(I1) amount in the Cu-doped a-FeOOH was obtained in the presence of bis(cyc1ohexane oxalyldihydrazone) from a HC1 solution of the sample. The Al(II1) content was determined with chrome azulen from a HC1 solution of the sample after extraction of Fe3+ with methyl isobutyl ketone. a-FeZO3and TiO, (anatase) prepared by decomposition of a-FeOOH at 673 K and Ti hydroxides at 873 K for 1 h in air, (1) Krylov, 0. V. Catalysis by Nonmetals; Academic Press: New York, 1970; Chapter 1. (2) Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985. (3) Verway, E. J. W.; Haaijman, P. W.; Romeijn, F. C.; von Oosterhout, G. W. Philips Res. Rep. 1950, 5 , 173. (4) Kroger, F. A. The Chemistry ofzmperfect Crystals; North-Holland: Amsterdam, 1964; Chapter 2. ( 5 ) Kofstad, P. Nomtoichiometry, Diffusion, and Electrical Conductivity in Binary Metal Oxides; Wiley-Interscience: New York, 1972; Chapter 2. (6) Morin, F. J. Phys. Rev. 1951, 83, 1005. (7) Lessoff, H.; Kersey, Y.; Horne, R. A. J . Chem. Phys. 1959, 31, 1141. (8) Nakau, T. J . Phys. SOC.Jpn. 1960, 15, 727. (9) Gardner, R. F. G.;Sweett, F.; Tanner, D. W. J . Phys. Chem. Solids 1963, 24, 1175. (10) Sanchez, H. L.;Steinfink, H.; White, H. S. J . Solid State Chem. 1982, 41, 90. (1 1) Benjelloun, D.; Bonnet, J.-P.; Dordor, P.; Launay, J.-C.; Onillon, M.; Hagenmuller, P. Rev. Chim. Miner. 1984, 21, 721. (12) Sieber, K. D.; Sanchez, C.; Turner, J. E.; Somorjai, G. A. J . Chem. SOC.,Faraday Trans. 1 1985, 81, 1263. (13) Kaneko, K.; Serizawa, M.; Ishikawa, T.; Inouye, K. Bull. Chem. SOC. Jpn. 1975, 48, 1764. (14) Ishikawa, T.; Inouye, K. J . Thermal Anal. 1976, 10, 399. (15) Kaneko, K.; Inouye, K. Bull. Chem. SOC.Jpn. 1974, 47, 1139. (16) Kaneko, K.; Inouye, K. Bull. Chem. SOC.Jpn. 1976, 49, 3689. (17) Kaneko, K.; Inouye, K. Bull. Chem. SOC.Jpn. 1979, 52, 315. (18) Kaneko, K.; Inouye, K. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 1258. (19) Sung, W.; Morgan, J. Enuiron. Sci. Technol. 1980, 14, 561. (20) Cornell, R. M.; Mann, S.; Skarnulis, A. J. J . Chem. SOC.,Faraday Trans. 1 1983, 79, 2679. (21) Paterson, R.; Rahman, H. J . Colloid Interface Sci. 1984, 98, 494. (22) Cunningham, K. M.; Goldberg, M. C.; Weiner, E. R. Photochem. Photobiol. 1985, 41, 409. (23) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986, 2, 203. (24) Ishikawa, T.; Nitta, S.; Kondo, S. J . Chem. SOC.,Faraday Trans. 1 1986, 82, 2401. (25) Kaneko, K.; Inouye, K. J . Chem. Technol. Biorechnol. 1987,37, 11. (26) Ishikawa, T.; Inouye, K. Nippon Kagaku Kaishi 1970, 91, 935. (27) Kaneko, K.; Inouye, K. Corrosion Sci. 1981, 27, 639. (28) Kaneko, K.; Inouye, K. Polyhedron 1984, 3 , 223. (29) Inoue, N.; Matsumoto, A,; Suzuki, T.; Ozeki, S.; Kaneko, K. Langmuir 1988, 4, 714. (30) Inouye, K.; Ishii, S.; Kaneko, K.; Ishikawa, T. Z . Anorg. Allg. Chem. 1972, 39, 8 6 .
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 1989
Valence-Controlled a-FeOOH
TABLE I: Crystallite Size of the (110) Plane, L , and Specific Surface Area, a I, of a-FeOOH as a Function of the Dopant Content
Ti/Fe. 5%
L,nm a.. m2/g
0
1.2
3.3
5.6
31 70
21 70
21 68
30 113
Cu/Fe, % L,nm a,, m2/g 1
20
I 40
I 60
2.3
3.0
3.5
6.3
22
20 170
0 235
0 290
300
120
J
, degree ( F e K a ) X-ray diffraction patterns of Ti-doped a-FeOOH.
respectively, were used in the photoadsorption experiments for comparison. Characterization. The X-ray diffraction patterns were obtained by the powder method with an automatic diffractometer (Geiger flex 2001, Rigaku Denki Co.) by use of Mn-filtered Fe K a (30 kV, 10 mA). The BET surface area was determined with a Shibata surface-area meter by using N 2 adsorption at 77 K. As for the Ti-doped a-FeOOH, the N 2 adsorption isotherms at 77 K were obtained by a conventional adsorption apparatus with a quartz spring. Electrical Measurements. Electrical conductivity of the powdered samples compressed between Au-plated phosphor bronze electrodes at 4 X lo6 kg m-2 was determined over the temperature range from 303 to 383 K and frequency range of dc, 1 kHz to 10 MHz under 10 mPa with a dc meter (Kawaguchi Denki, NA-l4A), an ac bridge made in this laboratory, and a Q meter (Meguro Dempa, M-l60B).lS The dc conductivity was measured at 100 V; the ohmic contact was ascertained by the proportionality of the current to the applied voltage. Steatites are used as insulating materials. The effect of oxygen of 20 kPa on the dc conductivity at 303 K was examined to establish the carrier type. The samples were pretreated at 373 K and 1 mPa for 14 h before the electrical measurement. SO2 Photoadsorptivity. The adsorption isotherms of SO2 on samples at 303 K were measured gravimetrically. The amount of irreversible adsorption after evacuation at 303 K and 1 mPa for 24 h was determined. The effects of illumination of nearUV-visible light (300-690 nm) by a Xe discharge lamp of 500 W (Wacom, KXL-SOOF) with a color filter (Toshiba Garasu, IRA-255) during 120 h on the SO2adsorptivity of the samples equilibrating with 33 kPa of SO2 were examined. The samples were pretreated as 373 K and 10 mPa prior to the adsorption experiments.
Results and Discussion Changes in Crystallite Size and Specific Surface Area. The X-ray diffraction patterns of Ti-doped a-FeOOH are shown in Figure 1. The X-ray diffraction peaks of a-FeOOH become lower with the Ti doping, and there is no shift in the peak position. Even a-FeOOH with Ti dopant of 5.6 at. % preserves the original crystal structure of a-FeOOH; there are no peaks due to Ti oxides. The introduction of Cu(I1) prevents a-FeOOH crystal formation; the sample containing 2% Cu(I1) has no peak, indicating an amorphous state, as previously reported.30 On the other hand, A13+ ions hardly change the X-ray diffraction patterns. The changes of X-ray diffraction patterns which the foreign ion dopants is explicitly shown in Table I. Table I shows the changes in the (1 10) crystallite size calculated from the Scherrer equation with various dopant contents. Both A13+and Ti4+scarcely change the crystallite size, while the crystallite size decreases with Cu2+ addition. Figure 2 shows the adsorption isotherms of N 2 on the Ti-doped a-FeOOH. The isotherms are of BDDT I1 type. All isotherms other than that of the 5.6% Ti-doped a-FeOOH nearly overlap. The relationships between specific surface area and the content of foreign ions are also presented in Table I. The specific surface area of Al-added a-FeOOH is almost constant, irrespective
0
AI/Fe, %
80
28
Figure 1.
1.4
L,nm as9 m2/g loo
1.o
3.0
5.0
32 68
32 12
31 60
'
7J G)
n
L
0 Ln
-u Q
40
c\1
Z
20
1 n
0
05
3.3%
10
PI Po
Figure 2. Adsorption isotherms of N2 on Ti-doped a-FeOOH.
8
c c
1OL
25
30
32
I / T , I/kK
Figure 3. Temperature dependence of the dc electrical conductivity of
Ti-doped a-FeOOH. of the A13+addition. The introduction of Cu2+increases sharply the specific surface area. In the case of Ti4+ions, the specific surface area maintains a constant value within the 3.3% doping and 5.6% Ti-doped a-FeOOH has a greater value. Electrical Conductivity, Figure 3 shows the temperature dependence of the dc electrical conductivity of a-FeOOH containing various amounts of Ti4+ions. All the logarithm of the conductivity
Kaneko et al.
1990 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
10
et
la L
m.12
1 4 i
14
I
2.6
I
1 3.0
I
2.8 l/l
,
I
I
3.2
-
-
tI
0
0
a
3.3
I1
? I '
9
5.6
i
dc
I/kK
3
Figure 4. Temperature dependence of the dc electrical conductivity of
Cu-doped a-FeOOH.
4
5
log t
, Hz
6
7
Figure 6. Frequency dependence of the electrical conductivity at 333 K of Ti-doped a-FeOOH. 1
1.0,
i
I
I
IO
20
?C
t l m e , min
Figure 7. Changes in the dc electrical conductivity of Ti- and Cu-doped a-FeOOH at 303 K upon the addition of oxygen gas of 20 kPa.
0
2
4
6
Dopant /Fe,atomic%
Figure 5. Changes in the dc electrical conductivity at 303 K and the activation energy for conduction with doping.
vs 1/T plots are linear except those in the higher temperature region. The 3.3 and 5.6% Ti dopants induce a more than 103-fold increase in the electrical conductivity. The temperature dependence of the dc conductivity of the Cu-doped a-FeOOH is shown in Figure 4. The electrical conductivity increases remarkably with the Cu content up to the 3.0% addition and then decreases at greater contents. Figure 5 shows the changes in the dc electrical conductivity a t 303 K and the activation energy for conduction obtained from the slope of the In Q vs 1/T plot with the dopant. The introduction of A13+does not affect the electrical conductivity or its activation energy, while Ti and Cu doping characteristically changes the electrical properties. It is noteworthy that the Ti doping leads to a clear effect on the conductivity. The activation energy is the hopping energy, which decreases with the carrier concentration due to the repulsion of carriers.I8 The fact that the activation energy decreases with the Ti and Cu contents must be caused by such a intercarrier interaction. Figure 6 shows the frequency dependence of the electrical conductivity of the Ti-doped a - F e 0 0 H at 330 K. These samples exhibit the conductivity dispersion described by the power law31332 gac a
P
Here Q, = uU, - crdc,fisthe frequency, and s is a constant. The s value in the frequency range of 1 kHz to 1 MHz decreases from (31) Pollak, M.; Geballe, T. H. Phys. Rev. 1961, 122, 1742. (32) Pike, G. E. Phys. Rev. 1972, 6B, 1572.
0.7 to 0.5 with increase of the Ti content. The conductivity dispersion is attributed to the electron hopping. The s values vary according to the mode of the hopping; s = 1 corresponds to single ~.~~ hopping, and s = 0.5 arises from m u l t i h ~ p p i n g . ~Therefore, the introduction of many Ti4+ions changes the hopping mode from the single process to the single and multi-mixed hopping process. The electrical conductivity of usual n-type oxides decreases upon adsorption of oxygen gas. The changes in the electrical conductivity of Ti (5.5%)- and Cu (4.0%)-doped a-FeOOH crystals upon the introduction of oxygen gas of 20 kPa at 303 K are shown in Figure 7. The ordinate of Figure 7 is the ratio of the electrical conductivity at a time to the original conductivity. The electrical conductivity of both samples decreases fast and seriously, indicating that the charged carriers in the Ti- and Cu-doped a-FeOOH are electrons. Mechanism of Valence Control in Hydroxides. The introduction of the foreign cations (Ti4+ or Cuz+ ions) of a valency different from the parent cations (Fe3+ions) gives rise to a distinct change in the electrical conductivity, as described above. Although Ti and especially Cu doping changes the crystallinity and crystallite size, the mixed valence states in a-FeOOH could be produced by their doping. Since the electron hopping depends only on the structure of the nearest neighbor in the lattice and does not require long-range order, the change in crystallinity is presumed to have no serious effect on the electrical conductivity; the remarkable conductivity change is mainly due to the carrier concentration rather than the mobility change. The change in the carrier concentration due to the doping can be interpreted by the following defect reactions. The defect reaction of the Ti-doped a-FeOOH may be written Ti(OH), = TieFe+ 4(OH)o + Fe'Fe (2)
Fe'Fe = Fe*Fe + e' (33) Pollak, M. Phys. Rev. 1965, 138A. 1822. (34) Lakatos, A . I.; Abkowitz, Phys. Rev. 1971, 38, 1791
(3)
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 1991
Valence-Controlled a-FeOOH 120
TABLE II: Monolayer Capacity and Amount of Irreversible Adsorption of SO2 on the Ti-Doped a-FeOOH without Photoillumination
- 100
. E" m
80
2
I
t5
60
L
irreversible adsorotion. . mdm2 -,
0
0.48
1.2 3.3
0.59 0.56 0.77
0.24 0.28 0.23 0.33
I
I
m
monolayer caDacitv. .., mn/m2 -,
Ti/Fe. 7%
v
,
5.6
I
0 lrl
2
40
,--
1.0
K-Fe2O3
N
0 Cn
20
. L
0 20
0
60
LO
.?
SO2 Equilibrium Pressure (kPa 1
Figure 8. Adsorption isotherms of SO2on Ti-doped a-FeOOH without photoillumination at 303 K.
Here we use the Kroger and Vink notation.35 TieFc,(OH)o, and FetFe show a Ti4+ion on the Fe lattice site, an OH- ion on the 0 lattice site, and an Fez+ion on the Fe site, respectively. The symbols *, ', and ' mean the zero, singly positive, and singly negative effective charges. The substitution of Fe(II1) with Ti(1V) produces quasi-free electrons after eq 3, in agreement with the experimental results. In the case of the Cu-doped a-FeOOH, two possible defect reactions may be considered: Cu(OH)* = Cu'pe Fe'F,
+ 2(0H)0 + Fe'Fe
+ e'
(4)
= Fe*Fe
+ 2e'
(6)
Here CuZis the doubly charged interstitial Cu2+ ion of positive effective charge. Since formation of the interstitial Cu2+ ions creates quasi-free electrons, the Cu doping is expected to enhance the electrical conductivity, agreeing with the experimental results for Cu additions until 3%. Therefore, the Cuz+ ions until 3% occupy interstital positions; excess Cu2+ions beyond 3% must be replaced with Fe3+ ions and work as acceptors. On the contrary, the defect reaction of Al(II1) introduction creates no charge carrier: Al(OH)3 = Al*Fe
+ (30H)o
(7)
The A13+addition, therefore, could slightly change the mobility at best; eq 7 is consistent with the electrical behavior of Al(II1). The ratio rM/rFc of the foreign ion radius to the Fe(II1) radius of the ionization energy change (AI) of the and the ratio N/AZFe dopant ion with the valence change to that (AIFc) of Fe(I1) Fe(II1) should be taken into account; both rM/rFe and N/AZFe should be nearly equal to unity to form a homogeneous solid values are 1.06 for Ti(IV), 0.80for Al(III), solution. The rM/rFe and 1.12 for C U ( I I ) . ~Ti4+ ~ ions are the most preferable to dissolve in the a-FeOOH lattice from the aspect of ion size. The AZ/AZFc values are 1.1 for Ti(II1) Ti(IV), 6.3for AI(II1) AI(IV), CU(III).~' Although Ti4+and Cuz+ are and 1.1 for Cu(I1) assumed to be able to work as an electron donor and acceptor,
-
- -
Q,
-2 0
4
0.L
0
0
Q N
0
m
02
0 ' 0
I
I
50
100
150
Irradiation Time ( h 1 Figure 9. Changes in the SO2adsorption with illumination at 303 K and
33 kPa SO2.
(5)
The quasi-free electrons annihilate according to eq 5, diminishing the electrical conductivity. Consequently, the above-mentioned reaction contradicts the experimental result, that is, the conductivity increases the Cu doping in the lower Cu(I1) concentration region. Another defect reaction may be written Cu(OH)2 = Cu2 + 2(OH)o
0.6
D
-
(35)Kroger, F.A.; Vink, H.J. In SolidStrrte Physics; Seitz, F.,Turnbell, D, Eds.;Academic Press: New York, 1956;Vol. 3, p 307. (36)Handbook of Chemistry and Physics, 54th ed.; CRC Press: New York, 1973;p E67. (37)Hundbook of Chemistry and Physics, 54th ed.; CRC Press: New York, 1973;p F194.
respectively, only the Ti4+ ion can be substituted with the Fe3+ ion, and the Cu2+ ion cannot mainly work as an acceptor but becomes an interstitial ion as mentioned before. The electrical conductivities of TiO,, CuO, and CuzO are low; also the electrical conductivities of Ti(OH)4 and Cu(OH), are presumed to be much lower notwithstanding the absence of their definite literature values. Thus the coprecipitated Ti4+and Cu2+ are assumed not to form the conducting path through the segregated hydroxides or oxides but to dissolve in the a-FeOOH lattice. The conclusion is that the coprecipitation of foreign ions in parent oxides can produce the mixed valence state in the case of hopping conduction systems. SO, Photoadsorptivity. Figure 8 shows the adsorption isotherms of SO, at 303 K without photoillumination. The amount of SOz adsorption per unit weight of the 5.6% Ti-doped a-FeOOH is much greater than that on other samples. The monolayer capacity determined by the BET plot and the residual amount of adsorption per the unit surface area after evacuation at 303 K and 1 mPa for 24 h, however, do not change with the Ti4+ content except for the 5.6% Ti-doped a-FeOOH, as shown in Table 11. Only the 5.6% Ti-doped a-FeOOH has a distinctly greater monolayer capacity (physical adsorption chemisorption) and irreversible adsorption (chemisorption) than the original aFeOOH. Although the increment of chemisorption is less than that of the monolayer capacity, the Ti doping should be associated with the enhancement of chemisorption. Figure 9 shows the changes in the SO, adsorption with photoillumination at 303 K and 33 kPa of SO,. As for a-FeOOH, the photoillumination brings about a slight decrease in the SO, adsorption, restoration from the adsorption drop and rise over the dark level. The 3% Ti-doped a-FeOOH shows the most marked photoadsorption after the initial photodesorption, whereas the 5.6% Ti-doped a-FeOOH hardly induces any photoeffect. The photoeffects of these pure and Ti-doped a-FeOOH are distinctly different from those of a-FezO3 and TiO,. Figure 10 shows the ratio of the SO2 adsorption after long photoillumination against the initial SO2 adsorption in the dark equilibrium state for each sample. Although we cannot determine the marked change in the SO2 adsorptivitiy in the dark, photoillumination leads to the characteristic change
+
1992 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
-
t
oxygen in the absence of water vapor to induce the photoadsorption. The coupling of the chemisorbed SO2- and a photogenerated hole could lead to its photodesorption as well as the photodesorption of oxygen molecules on ZnO and Ti02.42 Both formation of SO-: and disappearance of SO2-involve the process of electron migration; photoadsorption and photodesorption should be related to the electrical conductivity of the Ti-doped a-FeOOH. Furthermore, the SO2 chemisorption requires not only electrons but also accessibility, of the surface site to the adsorptive molecule. X-ray diffraction and specific surface area data show that the commensuration factor of the surface site for an SOz molecule could become less with the Ti doping, whereas the concentration of quasi-free electrons increases with the Ti doping. Hence the SO2photoadsorptivity change does not increase monotonously with Ti doping, but the 3.3% Ti-doped a-FeOOH probably has the greatest SO2 photoadsorptivity.
100
0
C
0
EL
9 +50
'0
N
0
0
m
c
0
a m
$
0
0
0
V K
-50 N-Fe203 0
2
L
6
Kaneko et al.
Ti02
Tl/Fe,alomic % T I -doped d-FeOOH
Figure 10. Rate in percent of change in SO2adsorption with photoillumination compared to the dark adsorption amount for various Ti-doped a-FeOOH.
in SO2 adsorptivity with the Ti doping. Bard and Frank38 showed that SO?- is oxidized to SO-: by reaction with photogenerated holes of TiOz and ZnO suspended in water. It is known that an adsorbed SO2accepts an electron in the photoirradiated lattice of TiOz and MgO at the initial stage SOzmolecules of sulfur oxide radical f ~ r m a t i o n . ~The ~ *adsorbed ~ attract electrons from a-FeOOH on chemisorption in the dark to become the SO2-species as shown by electrical conductivity measurements?' The adsorbed SOzreacts with the surface oxygen of a-FeOOH to form the SO?--like species through SOz-without photoillumination according to a recent FTIR e ~ a m i n a t i o n . ~ ' Photoillumination is presumed to accelerate the formation of the S0,2--like species on the a-FeOOH surface with the use of surface (38) Frank, S.N.; Bard, A. J. J . Phys. Chem. 1977, 81, 1484. (39) Gonzalez-Elipe, A. R.; Soria, J. J . Chem. Sor., Faraday Trans. I 1986, 82, 739. (40) Lin, M. J.; Lunsford, J. H. J . Phys. Chem. 1975, 79, 892. (41) Matsumoto, A,; Kaneko, K. Abstract of 40th Colloid Interface Chemical Symposium, Kyoto, Japan, 1A12.
Conclusion
The hydrolysis of the mixed Fe3+-Ti4+solution leads to Ti-doped a-FeOOH without collapsing the lattice structure of a-FeOOH; the Ti doping enhances the n-type electrical conductivity of aFeOOH. A Cu2+ ion mainly occupies the interstitial position of the a-FeOOH lattice and breaks the lattice structure, inferred from the marked increase of n-type conductivity and noncrystallization due to the Cu doping. In hopping conduction systems, noncrystallization hardly prevents the conductivity from increasing on doping with Cu2+ ions. The valence state of a-FeOOH can be controlled by the Ti or Cu doping with the coprecipitation method. On the other hand, the addition of AI3+cannot control the valence state of a-FeOOH. The SO2photoadsorptivity of the Ti-doped a-FeOOH depends on the Ti4+content, while the dark SO2chemisorptivity scarcely changes with Ti-doping. It is suggested that quasi-free electrons due to the Ti doping produce SO2-and accelerates the successive oxidation of SO2- to S032-on the surface. Acknowledgment. We thank Professor Emeritus Katsuya Inouye for valuable suggestions. Registry No. FeOOH, 20344-49-4; 02,7782-44-7; SO2,7446-09-5; Cu, 7440-50-8; Ti, 7440-32-6; Al, 7429-90-5. (42) Cunningham, J.; Hodnett, B. K. J . Chem. SOC.,Faraday Trans. 1 1981, 77, 2777.
