Ind. Eng. Chem. Prod. Res. Dev. 1988, 25,
93-96
Photoeffects Due to Thickness and Dopant (Sb,O,) Polycrystalline TiO, Electrodes
93
in
KI Hyun Yoon,' Dong Heon Kang, Keu Hong Kim,+ and Jae Shl Cholt Department of Ceramic Engineering and Department of Chemistry, Yonsei University, Seoul, Korea
The photoelectrochemical properties of reduced TiO, ceramic electrodes are investigated by varying the thickness of the electrodes and the amount of Sb2O3 dopant. As the thickness of the undoped TiO, ceramic electrode increases, the photocurrent tends to decrease. However, for R-F sputtered TI02 thin-film electrodes, the photocurrent tends to increase with increasing thickness to about 1 pm and then decreases with increasing thickness. For TiO, ceramic electrodes doped with Sb203, the photocurrent decreases with increasing amounts of dopant, and visible light excitation is observed at wavelengths of -500 nm compared with -420 nm for the undoped TiO, ceramic electrodes.
Introduction Photolysis of water utilizing a semiconductor-electrolyte junction has been under study as a possible efficient method for utilizing solar energy. Boddy (1968) observed that holes from TiOz can oxidize water; and Fujishima and Honda (1972) first suggested that water could be photoelectrolyzed into hydrogen and oxygen in a photoelectrochemical (PEC) cell composed of an n-type Ti02 anode and a Pt plate cathode. Butler (1977) and Wilson (1977) have explained the PEC conversion mechanism using the Schottky barrier model, and a previous paper from this laboratory (Yoon, 1984) reports on the PEC properties of polycrystalline Ti02 where electrodes were prepared by flame oxidation of the titanium. These studies have shown that the major problem is to select suitable semiconducting material for the photoelectrode and that from practical and economical considerations it would be favorable if the polycrystalline electrode could be employed effectively in the fabrication of these photoelectrodes. In this work, among the factors influencing the PEC conversion, we investigate the effects of the thickness of the electrode using polycrystalline TiOz electrodes prepared by two different techniques: sintered Ti02ceramic electrodes and R-F sputtered TiO, thin-film electrodes. Moreover, in hope of improving the sensitivity of Ti0, to wavelengths in the visible spectrum,as it has a large energy band gap (-3.0 eV), the PEC properties of the TiOz ceramic electrode doped with Sbz03are studied in terms of defect reactions. Experimental Section The raw materials for the ceramic specimens were TiOz rutile powder (purity 99.99%, High Purity Chemicals Lab., Japan) and Sb203powder (purity 99.970,Merck Co., W. Germany), which were mixed in ethanol in the proportions shown in Table I. The obtained powders were pelletized and then sintered in air at 1250 "C for 90 min. In order to have electrodes of different thicknesses, from 100 to 400 pm thick, the sintered pellets were sliced with a low-speed cutting machine, and one surface was polished, step by powder. All of the step, with Sic papers and 0.3 pm A1203 specimens were then reduced under hydrogen flow, controlled by manometer, for 1h at 800 "C, the conditions for which the undoped TiO, ceramic electrode showed the best Department of Chemistry. 0196-4321/86/1225-0093$01.50/0
Table I. Composition of Specimens raw material, wt % specimen no. TiO, Sb,O, TP 100 0 TS-1 99.75 0.25 TS-2 99.50 0.50 TS-3 99.00 1.00 TS-4 98.00 2.00
photoeffect. The reduced specimens were cleaned with acetone and distilled water. After the specimens were dried, a copper wire was attached by conducting Ag cement. The electrode and wire were then sealed over with epoxy resin (Devcon Corp.) except for the polished surface of the electrode. Current-voltage curves for the reduced Ti02-Ag cement electrode junctions were found to follow Ohm's law. The TiO, thin films were deposited on Ti substrates by a R-F sputtering machine (MRC Model 3665). A sintered pellet of rutile (0.d. 6 cm, purity 99%, Kanto Chemical Co., Inc.) was used as the target. A Ti metal rod (0.d. 12 mm, purity 99.97%, Materials Research Corp.) was cut into about 0.5-cm lengths, and one surface was polished with Sic paper and A1,0, powder. After the rods were cleaned with acetone and distilled water, they were used as the substrates. During sputtering, the temperature of the Ti substrate was maintained at about 200 "C to ensure a uniform film without cracks. The film obtained by the above procedure was reduced in a hydrogen atmosphere at 800 "C for 3 h in order to decrease its resistance. This treatment caused a dark gray coloration of the surface of the film like that of the TiO, ceramic specimen. X-ray analysis revealed a diffraction pattern indicating a rutile structure. The thickness of the TiO, films was measured by an A-interferometer (Varian Model 346) and ranged from 0.1 to 1.5 pm. Electrical contact and seals were made on the Ti side of the specimen, as described above, and the TiO, side exposed to the electrolyte. The PEC measurements were performed in 1N NaOH electrolyte by using the three-electrode PEC cell described by Yoon et al. (1984) with a TiOz anode, a Pt plate cathode, and an Ag-saturated AgCl electrode (Pope Science, Inc.) as the reference electrode. The light source was a 150-W W-halogen lamp (Hanimex Projector Lamp, W. Germany). Photocurrent-potential behaviors were measured in terms of the current variation between the TiO, anode and the Pt cathode vs. the potential drop across a 1004 resistor 0 1986 American Chemical Society
94
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986
* 01 um thickness x03um
11
100 Current
1
(uA)
Liaht
Dork
‘Potential ( V \is Ag/AgCll
Figure 1. Current vs. applied-potential curves for undoped TiOz ceramic electrodes.
as the potential was varied between the TiOz anode and the reference electrode. The input potential was varied with an external variable power supply. For photoexcitation experiments, a monochromator (Bausch & Lomb Model 33-86-79) with a frequency range of 200-800 nm was installed at the light source to provide monochromatic output to the junction, and a radiometer/photometer (EG & G Inc. Model 550) was used to determine the incident light intensity. The photocurrent, was observed by electrometer (Hewlett-Packard Model 427A) for each wavelength. The potential variation between the TiOz anode and the reference electrode was measured by digital multimeter (Keithly Model 177). Results and Discussion When the semiconducting electrode is immersed in the electrolyte and illuminated, it is likely to be most photoactive if the optical absorption depth corresponds closely to the depletion-layer width formed by the semiconductor-electrolyte junction (Butler, 1977). Furthermore, if the thickness of the semiconducting electrode becomes comparable to the depletion-layer width, the photoexcited carriers within the depletion layer will be utilized more efficiently. Figure 1 shows the photocurrent-potential behaviors for undoped T i 0 2 ceramic electrodes with thicknesses from 100 to 400 pm. According to Butler’s work (19771, the depletion-layer width, W , in a photosensitive semiconducting electrode is given by
where q,is the permittivity of free space (8.85 X lo-’, F/m), t is the relative dielectric constant of the semiconductor, Nd is the donor density, V is the applied potential relative to the reference electrode, and V , is the flat-band potential. So, the depletion-layerwidth depends on the donor density and the applied potential. The optical absorption coefficient, which is given by (hv - Eg)n’2 N = A(2) hu
depends on the photon energy (hv),the energy band gap of the semiconductor (E,),.and n ( n = 1for direct allowed transitions, n = 4 for indirect transitions). In this work, as all of the TiO, specimens were prepared under the same reduction treatment and illuminated by the same light source, it can be assumed that the values of the donor density and the optical absorption coefficient are nearly constant. Therefore, the values of the optical absorption depth and of the depletion-layer width (W) should be nearly constant at a given potential according to eq 1 and 2.
Potential ( V vs Ag/AgCl) Figure 2. Current vs. applied-potential curves for sputtered TiOz thin-film electrodes.
Salvador (1984) has reported that the hole diffusion length, L,, is reduced due to the disturbed surface layer introduced by the mechanical polishing, the accumulation of lattice defects, and the segregation of impurities at grain boundaries which behave as recombination centers. In the case of our experiments performed with mechanically polished TiOz ceramic electrodes, the L, values are smaller than for TiO, single crystals and very small compared with the depletion-layer width of the TiOz ceramic electrode. Thus the photocurrent is primarily due to carriers generated in the depletion layer (Butler, 1977). In regard to the PEC energy conversion, the change in density of the total current depends on the optical absorption coefficient ( a ) ,the depletion-layer width ( W), and the hole diffusion length (L,) according to the equation (3)
If we introduce the above quantities from our experiments into eq 3, the density of the total current should be constant, independent of the thickness of the electrode. However, in the case of the ceramic electrode, the thickness of the electrode itself corresponds to the thickness of the oxide layer, which is thicker than the depletion-layer width formed in the electrode. And as the thickness of the ceramic electrode increases, the photoexcited carriers have to move a relatively longer distance to the contact (Mollers et al., 1974). Therefore, as shown in Figure 1,the photocurrent tends to decrease with increasing thickness of the undoped TiOpceramic electrode due to the increased oxide layer, which behaves as a barrier to movement of the carriers. But the dark current, which is also shown in Figure 1,was less than 10 pA, independent of the thickness of the TiO, ceramic electrode. In Figure 2 are given the photocurrent-potential curves for the sputtered TiO, film electrodes from 0.1 to 1.5 pm thick prepared by varying the deposition time. As the film becomes thicker, up to about 1 pm, the photocurrent gradually tends to increase and then decreases with increasing thickness. The dark current is practically nil. As the film is thick enough to allow the development of a full depletion layer, the photoexcited electron-hole pairs will be more efficiently separated and utilized due to the increase in absorption of light in the deletion layer. Thus the photocurrent should increase with increasing thickness. However, above about 1.0 pm, the photocurrent tends to decrease. This might be explained if the oxide film layer above moderate thickness comparable to the depletionlayer width behaves as a barrier to movement of the photoexcited carriers. Figure 3 shows the photocurrent-potential behaviors for Sb,O,-doped TiO, ceramic electrodes with the composi-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986 Ti 0, XTIO,+OE% Sb203
w
1
500 Current
x
(uA)
~I0~+025&03
T101+05 'hSbz03 quenched in a i r - - - x T102+Q25~mzO, ~ T 1 0 2 + 0 y16Sb203 5 A
95
500-Current (uA)
I . . . . , . , 05
-05
0 1
Potential ( V vs Ag/A&ll
Figure 3. Current vs. applied-potential curves for undoped and Sb,O,-doped TiO, ceramic electrodes.
tions of Table I. The photocurrent decreases as the amount of Sb203increases. To decrease their resistance, the Ti02 ceramic specimens were reduced in a hydrogen atmosphere. This treatment causes oxygen vacancies and Ti3+donor ions in the Ti02structure to follow these defect reactions: Oo = vo"
+ 2e + ' j 2 O 2
Figure 4. Current vs. applied-potential curves for reduced and quenched TiO, ceramic electrodes with Sbz03.
TiO, TI 0, + 0 25 w/oSb 0, oTiO,+ I O U h S b , 4 x
I-
(4)
Ti4+ + e = Ti3+
(5) In Sbz03-doped Ti02 ceramics, the Sb203is dissolved substitutionally in the Ti02,producing oxygen vacancies as follows: Sb203 = 2SbTi'
+ VO" + 3 0 0
(6)
The resulting dark gray coloration is like that of the reduced TiO, ceramics. Maxwell (1963) has stated that at high temperature TiO, comes to an equilibrium with a deficiency of oxygen and that while this oxygen deficiency is normally eliminated during cooling, antimony may possibly prevent the combination of Ti02 with the required oxygen. So it is suggested that in the case of Sb203-dopedTi02, an oxygendeficient conductive specimen could be produced. Under the same reduction treatment, the concentration of oxygen vacancies in a Sb203-doped TiO, ceramic electrode is possibly higher than that in undoped TiOz. Furthermore, it has been reported that oxygen vacancies can serve as traps and recombination centers which impede the separation of photogenerated electron-hole pairs and promote their recombination (Subbarao et al., 1978). Thus this gradual decline in photocurrent as the amount of Sbz03 ranges from 0.25 to 2.0 w t 7% in Figure 3 can be attributed to the increasing oxygen vacancy concentration. Also, through eq 1and 3, the decrease in totalphotocurrent can be explained as a consequence of the decreased width of the depletion layer resulting from the increase in donor density. For air-cooled Ti02ceramic electrodes doped with Sb203 without the reduction treatment, we still observe a photocurrent, though lower than that for the reduced TiOz, as shown in Figure 4. The appearance of this photocurrent can be explained as also being due to Ti3+donor ions related to the formation of oxygen vacancies in the TiO, structure induced by Sbz03doping following eq 4-6. For the undoped TiO, ceramic electrode without reduction treatment, however, no photocurrent was observed in the case of air-cooling treatment. Figure 5 shows the spectral dependence of the quantum efficiency for both undoped Ti02 and Sb203-dopedTi02 ceramic electrodes at an anode potential of 0 V vs. Ag/ AgCl. The quantum efficiency q (defined as the number of electrons flowing/ incident photons) was calculated by using measured values of the photocurrent density and
-2 Wavelength (nm)
Figure 5. Quantum efficiency vs. wavelength curves for undoped and Sbz03-dopedTiO, ceramic electrodes under 0 V vs. Ag/AgCl.
light intensity. For the undoped TiO, ceramic electrode, the maximum value of 9 is about 0.15 at 340 nm, which is even lower than that for single-crystal Ti02 (Desplat, 1976). However, the photoresponse appeared around 420 nm, which nearly corresponds to the energy band gap of TiOz (-3.0 eV). This agrees also with the photoresponse of single-crystal TiOz (Hardee and Bard, 1975) and polycrystalline TiOz (Yoon, 1984). A shift of photoresponse toward longer wavelengths (-500 nm), with respect to the undoped TiO,, was observed for the Sb203-dopedTi02 ceramic electrode as shown in Figure 5. Matsumoto et al. (1980) reported that for a polycrystalline Ti/CoO, electrode a visible light photoresponse (500-600 nm) was observed due to the subband formed by the interaction between d orbitals of Co ions in the lattice. In the case of Sb20,-doped Ti02 ceramic electrodes, the visible light photoresponse (-500 nm) might be attributed to the movement of excited electrons through the subband formed by the interaction between the d orbitals of Ti ions and the p orbitals of Sb ions in the TiOz lattice. Conclusions (1)As the thickness of the undoped Ti02ceramic electrode increases, the photocurrent tends to decrease. However for R-F sputtered TiOz thin-film electrode, the photocurrent tends to increase with increasing thickness to about 1 pm and then decreases with increasing thickness. (2) For the Ti02 ceramic electrode doped with Sb203, the photocurrent decreases with increasing amounts of dopant due to the increased concentration of oxygen va-
Ind. f n g . Chem. Prod.
96
Res. Dev. 1986, 25,96-102
cancies compared with the undoped Ti02. (3) A shift of the photoresponse toward longer wavelength (-500 nm) is observed compared with that for the undoped TiOz ceramic electrodes (-420 nm). Acknowledgment This work was supported by the Korea Traders Foundation. We thank Professor Robert G. Sauer for valuable comments on the manuscript. Registry No. Ti02,13463-67-7;Sbz03,1309-64-4;Ti, 7440-32-6. Literature Cited
Desplat, J. L. J . Appl. Phys. 1978, 4 7 , 5102. Fujishima. A.; Honda, K. Nature (London) 1972, 2 3 8 , 37. Hardee, K. L.; Bard, A. J. J. Electrochem. SOC. 1975, 122, 739. Matsumoto, Y . ; Kurimoto, J.; Amagasaki, Y.; Sato, E. J . Nectrochem. SOC. 1980, 127, 2148. Mollers, F.; Tolle, H. J.; Memming, R . J . Nectrochem. Soc. 1974, 121, 1160. Maxwell, L. H. Ceram. Bull. 1963, 4 2 , 438. Salvador, P. J . Appl. Phys. 1984, 55, 2977. Subbarao, S. N.; Yun, Y . H.; Kershaw, R.; Dwight, K.; Wold, A. Mater. Res. Bull. 1978, 13, 1461. Wilson, R . H. J . Appl. Phys. 1977, 4 8 , 4292. Yoon, K. H.; Yoon, S. 0. Jpn. J . Appl. Phys. 1984, 23, 1137. Yoon, K. H.; Kim, J. S.Jpn. J . Appl. Phys.. in press.
Received for review April 2 , 1985 Accepted August 30, 1985
Boddy, P. J. J . Nectrochem. Soc. 1988, 115, 199 Butler, M. A. J. Appl. Phys. 1977, 4 8 , 1914.
Syntheses of Derivatives of Alkylarylamines and Their Properties as Pickling Inhibitors of Carbon Steels and Stainless Steels Marek Studnlckl Institute of Inorganic Chemistry, 44- 101 Gliwice, Sowifisskiego 11, Poland
Preparation and application of pickling inhibitors derivatives of alkylarylamines in hydrochloric acid and sulfuric acid are described.
Introduction Application of corrosion inhibitors of metals in strongly corrosive media is the cheapest and best way to prevent corrosion. In recent years derivatives of alkylarylamines were found to be among the most effective inhibitors, and because of their good properties, they can compete with cheaper but less active inhibitors. The aims of this study were syntheses of (chloroalky1)arylamines and investigations of their properties (Studnicki, 1983, 1984, 1985). A new method of synthesizing monosubstituted derivatives of hydrazine that involves reaction of (chloroalky1)arylamines with hydrazinium carbonate was elaborated. This new approach gives higher yields of mono derivatives than by presently known methods (Ioffe et al., 1979; Osei-Twum et al., 1984; Pilgram and Skiles, 1983). These compounds were not isolated in these investigations as corrosion inhibitors. Their purification and analyses will be described in future publications. Hydrazine is readily alkylated by heating with haloalkanes, but it must be realized that alkylation can proceed further to form quaternary salts. Thus, from HzNNHzand RX one obtains not only RHNNH2but also RzNNH2and R3NNH2X-. One cannot obtain 1,2-dialkylhydrazines in this way. In order to convert hydrazine into a monoalkylhydrazine it is necessary to use a large excess of hydrazine to prevent further alkylation. If the R in RX is primary alkyl, it requires greater dilution than if R is secondary, as from 1O:l for ethylhydrazine to 4:l for isopropylhydrazine. To synthesize higher monoalkylhydrazines one must use alcohol as solvent; without it, only 1,l-dialkylhydrazines are obtained (Ioffe et al., 1979). A new synthesis of salts of S-(aminomethyl)- or S-(diaminomethy1)isothioureas was devised by reaction of N-chloromethyl o r N-dichloromethyl compounds with thiourea in the presence of stannous chloride. Known 0196-432118611225-0096$01.50/0
Scheme I' Ar-NH,
+ C1-R-CI
ArNH(R-C1)
+ H-B
A
-
SnC1,
t H' --+ (Cl-R)+NH,Ar
+
HCl
ArNHR-B.HC1
H-B = H-NH,, H-NH-DBA, H-NHNH,, H-SC(NH)NH, B
c
N
S
RCI, = methylene chloride, chloroform; Ar = 0-,m - , p-phenylene; 2, 3,4-dichlorophenyl; 3, -phenylf 4, 3chlorophenyl; 5 , 3-hydroxy phenyl; 6, 3-methoxyphenyl. A, Table I ; B, Table 11; C, Table 111; N, Table IV; S, Table V.
methods of preparation of these compounds rely on reaction in solvents (Autorenkollektiv, 1971; Freidlina et al., 1983; Pandeya and Ram, 1981). In thiourea the sulfur atom has strong electrophilic properties, and the basic nitrogen atoms increase the electron density on the sulfur, which means that the aliphatic halogens and thiourea easily form a salt: (a1iph)-C1 + SC(NHJ2
SnC1,
(a1iph)-S-C(NH)-NH, + HC1 (as salt)
Based on published information, the most commonly used corrosion inhibitors are meta-substituted anilines and derivatives and benzimidazole (Desai and Shah, 1972; Singh et al., 1972; Desai and Thanki, 1972; Desai, Gandhi, and Joshii, 1974; Patel and Franco, 1975;Talati and Patel, 1978; Aal and Abdel Assaf, 1980; Desai and Desai, 1981; Trabanelli et al., 1973; Berthold et al., 1978; Lomakin, 1979; 1980; Anderson, 1980; McCrory-Joy and Rosamila, 1982). Acidic pickling to clean the surfaces of apparatuses and devices is connected with corrosion of metals, and one must 8 1986 American Chemical Society
