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Structure and photoelectrochemical efficiency of oxidized titanium electrodes
Int. J. Hydrogen Energy, Vol. 7, No. 10, pp. 769-774, 1982. Printed in Great Britain.
0360-3199/82/1007694)6 $03.00/0 Pergamon Press Ltd. 1982 International Association for Hydrogen Energy.
S T R U C T U R E A N D P H O T O E L E C T R O C H E M I C A L EFFICIENCY OF O X I D I Z E D TITANIUM E L E C T R O D E S V.
ANTONUCCI,*
N. GIORDANO* and J. C. J.
BARTt
*Istituto C. N. R. di Ricerche sui Metodi e Processi Chimici per la Trasformazione e l'Accumulo dell'Energia, Pistunina, Messina, Italy; tIstituto G. Donegani, Via G. Fauser 4, 28100 Novara, Italy
(Received for publication 29 December 1981) Abstraet--Pure titanium laminae were oxidized in air between 300 and 1300°C and used as electrodes in the photodecomposition of water. The maximum photoelectrochemical conversion was found for flame-oxidized (1300°C) samples. Although the efficiency shows a parallelism with the presence of the metallic interstitial compounds TiO0+x (x < 0.33) and Ti20~-y (0.33 > y > 0) at the metal-semiconductor interface, the thickness of the suboxide layer and that of the external rutile scale, it is argued that other factors (optimal suboxide-rutile distribution, overall layer thickness, layer perfection, non-stoichiometry of TiO2, isoelectric point of solids (IEPS), etc.) may be more important in the process.
(anatase, brookite, h. p. form), TiO (ASTM 23-1078) and Ti,O2,- 1 (where n = an integer) were not detected. In order to relate the structure and efficiency of photoelectric conversion, the electrodes (0.5 cm) were immersed in a homogeneous photoelectrochemical cell containing a liquid electrolyte (0.1 N Na2SO4), a Pt counter-electrode (Engelhard, 15 cm2), a current meter (Keithley Multimeter) and an external bias device. A 400W mercury lamp (Osram), emitting in the 3101000nm range, was used as the light source. The efficiency
INTRODUCTION Titanium dioxide is widely used in the photosensitized electrolysis of water because of its non-corrosive properties, compared to other semiconductors, in spite of the high band-gap (3.02 eV) which limits its efficiency in the conversion of irradiated visible light into chemical energy. TiO2 is often used in single crystal form [1], or more recently also as polycrystalline material obtained from titanium compounds [2], deposition or evaporation as thin films [3] and electrochemical [4] or thermal oxidation of metallic sheets [5]. As very little [6] has been reported on the influence of the preparation of such devices on their energy balance and structure, in this paper we study the thermal oxidation of metallic laminae in relation to their performance as photoelectrodes.
r/= Ic(1.23 - V) × lO0/Wa was calculated according to Ghosh and Maruska [9], where Ic is the cell current (mA cm-2), V the bias and W, the energy content of incident light (measured with an electrosolarimeter, Sun System F D R 5003 B), in conditions with (0.2 and 0.5 V) and without bias.
EXPERIMENTAL As the oxidation kinetics of titanium and titaniumbased alloys is influenced by a variety of factors, for example the nature of the gas phase, the surface treatment of the metal and its chemical composition [7], pure titanium laminae (VT1, 99.5% Ti; principal impurities Fe, Si, C; 0.5 mm thick) were oxidized in controlled conditions, namely in air without pretreatment of the surface. The material was exposed to air in an electric furnace between 300 and 1000°C for various lengths of time (from 15 min up to i h in 15 rain intervals). A second series of electrodes was flameoxidized at 1300°C for 1-20 min. Phase analysis was carried out at room temperature in reflectance geometry by directly mounting the electrodes on a carefully calibrated X-ray powder diffractometer (Philips) using CuKo:radiation (). = 1.5418/~). The spectra were interpreted with reference to rutile (ASTM 21-1276), Ti (ASTM 5-0682), Ti20 (ASTM 11-218) and Ti30 (ref. [8]); other polymorphs of TiO:
RESULTS No new (bulk) phase formation in the electrodes was detected by X-ray diffraction methods up to 500°C (see Table 1). Small amounts of rutile were observed after oxidation in air at 600°C for 15 min; the TiO2 layer thickness increased with reaction time. Rutile formation conforms with previous results [10--13]. In the sample oxidized at 600°C for 60min, a titanium suboxide (TiO0+x, x < 0.33) [14] was detected by line-broadening at the low angle side of the oriented Ti (002) reflection. At 700°C the effects were more pronounced with increased formation of the two oxides; after activation for 60 min at this temperature line-broadening was also detected at the weak Ti (012) reflection. At 800°C the characteristic reflections of metallic titanium strongly decreased in intensity, especially after prolonged oxidation (1 h), thus indicating the presence of a fairly thick oxide layer (estimated thickness 10-15 #m). The non-oriented rutile scale grew rapidly at this tempera-
769
V. ANTONUCCI, N. GIORDANO AND J. C. J. BART
770
Table 1. Phase analysis of oxidized titanium electrodes* Temperature (°C) 300-500 600 700 800 900
1000 1300
Time (min)
Ti
15-60 1545 60 15-60 15-30 45 60 155 60~: 15§ 30-60§ 15-45:~§ 605 1-2 3-5 20
O O O O + + O + t
Phase distribution TiO0÷~ Ti30 + + + + + + t
+ + + + + + + + +
Ti2Ol-y
TiO2
+ + ++ ++ -
+t + + + + + + + + + + + O + + +
* Results of XRD, unless otherwise indicated. 5 Surface structure (XPS). ~: Before flaking; o, epitaxial orientation. § After flaking. t, traces; +, present; - , absent.
ture and TiO2 was the main phase after 1 h at 800°C when metallic titanium was completely X-ray shielded. A t increasing reaction times at 800°C the diffraction bands of TiO0 +x gradually shifted away from the angular positions of metallic titanium. The maximum suboxide layer thickness was reached after 45--60 min at 800°C. In the 900°C samples the average suboxide (002) position is 37.70 ° (d = 2.386 ]~), which is to be compared to those of Ti (38.430°), Ti30 (37.730 °) and Ti20 (37.123°). Oxidation at 900°C leads to poor adherence of the oxide surface layers to the grey metal core (cf. also ref. [15]). This indicates considerable stress at the interface, probably the result of the volume change accompanying the conversion of metal to oxide [10, 16]. Before the white surface scale flakes off the core, the X-ray spectra of the 900 ° samples show mainly futile together with some Ti30. Afterwards, the layer consists of a thin residue of TiO2 on top of a thick Ti30 substrate. The titanium core was never brought out in the spectra of the 900°C samples. A t higher oxidation times (up to 1 h) the increasing (002) line asymmetry indicates the presence of a second suboxide (Ti2Ol-y with y < 0.33 or a Ti20 phase with an oxygen deficit) in minor amounts at c a 37.35 ° ( d = 2.408A). A f t e r deconvolution the gradual narrowing of the (002) reflection with activation time (hla values of 0.40 ° and 0.30 ° after 15 and 60 min, respectively) indicates either an increase in crystallite size or a more h o m o g e n e o u s oxygen dissolution. Essentially similar results were found at 1000°C. These electrodes have a white-grey aspect, indicating a fair degree of adherence of the (residual) rutile layer to the substrate. Two Ti2Ol-y phases were clearly distinguishable by the (002) reflections at 37.59 ° and 37.28 °
after oxidation for 15 min, and at 37.45 ° and 37.24 ° after 60 rain. The flame-oxidized samples (1300°C) are in line with the previous picture and show increasing amounts of TiO2 (rutile) on a Ti substrate (detectable in the X-ray spectra) in the presence of substantial quantities of TiO0+x(x < 0.33) and Ti30. The Ti30 layer thickness is almost constant during the initial oxidation period (1-4 min) and is slightly inferior to that in the sample oxidized at 800°C for 45 min. A f t e r prolonged heating the suboxide layer thickness decreased strongly and a thick oriented rutile layer was observed instead. The Ti30 crystallite size (measured for the (002) plane) increased from c a 160 to 300/~ during the oxidation process (1-20 min). No Ti reflections were observed in the X-ray spectrum of the sample flame-oxidized for 20 min, as opposed to TiO2, Ti30 and a small residue of TiO0+x as a shoulder of the Ti30 (002) reflection at the high angle side. The slight, but significant, difference in the suboxide (002) lattice position before and after loss of the rutile surface layer at 900°C (from 37.69 ° to 37.74 °, 20) indicates the variable composition of the scale, which is tess oxygen-rich at greater depth, in accordance with the expectation. A t higher temperatures (cf. 1000 ° and 900°C) m o r e oxygen is dissolved, as expressed by the shift of the (002) reflection to lower 20 values, and one of the Ti201 -y phases approaches the Ti20 composition. The photoelectrochemical efficiency (r/%) for titanium electrodes oxidized between 300 and 1000°C in the furnace and those flame-oxidized at 1300°C are given in Tables 2 and 3, respectively, for various values of the external bias. U p to 800°C the most efficient
771
OXIDIZED TITANIUM ELECTRODES Table 2. Photoelectrochemical efficiency (~/%) for titanium electrodes oxidized between 300 and 800°C at various values of the external bias t=15'
%
t=30'
%
T (°C)
V= 0
0.2
0.5
0
0.2
300 400 500 600 700 800
0.01 0.015 0.064 0.083 0.241 0.317
0.01 0.025 0.068 0.084 0.227 0.309
0.017 0.028 0.067 0.073 0.176 0.268
0.013 0.024 0.088 0.098 0.275 0.076
0.014 0.039 0.091 0.107 0.262 0.082
t=45'
0.5 0.023 0.045 0.101 0.105 0.217 0.080
electrodes were those oxidized at higher temperatures for gradually lower reaction times. The maximum efficiency was found for a flame-oxidized (1300°C) sample, in accordance with the literature [6]. Obviously, under these conditions the formation of a non-stoichiometric titanium oxide layer with good semiconducting properties requires very short oxidation times; at prolonged exposure to air the stoichiometric compound TiO2 is formed, which is quite unfavourable for photoelectrolysis. Essentially similar observations apply to the 300-800°C range. Due to flaking, application of the laminae oxidized at 900-1000°C in the photoelectrochemical cell was not considered. DISCUSSION
Physico-chemical characterization Thermodynamic considerations indicate that TiO2 is the most probable crystalline product of titanium oxidation under a variety of conditions of gaseous atmosphere (air, steam, CO2), temperature and time, according to the irreversible processes Ti(s) + O2(g)
--+ TiO2(s)
(1)
Ti (s) + 2H20 (g) --+ TiO2 (s) + 2H2 (g)
(2)
Ti (s) + 2CO2 (g) -+ TiO2 (s) + 2CO (g).
(3)
However, in spite of a fair degree of agreement in Table 3. Photoelectrochemical efficiency (r/%) for titanium electrodes flame-oxidized at 1300°C at various values of the external bias
r (oc) 130o t 1' 1' 30" 2' 3' 4' 5' 10' 20'
%
V= 0
0.2
0.5
0.7
0.9
0.45 0.49 0.74 0.57 0.37 0.33 0.32 0.15
0.45 0.51 0.65 0.52 0.38 0.31 0.31 0.14
0.39 0.46 0.51 0.44 0.34 0.27 0.27 0.12
0.33 0.39 0.46 0.42 0.39 0.23 0.23 0.10
0.33 0.30 0.36 0.37 0.30 0.18 0.18 0.10
%
0
0.2
0.022 0.027 0.039 0.130 0.300 0.074
0.033 0.037 0.049 0.144 0.295 0.095
t=60'
0.5 0.055 0.041 0.048 0.114 0.255 0.100
%
0
0.2
0.5
0.015 0.032 0.032 0.172 0.177 0.044
0.027 [).033 0.035 0.175 0.189 0.045
0.031 0.041 0.034 0.174 0.178 0.044
experimental oxidation rates, phase compositions and proposed oxidation mechanisms differ widely [17-19]. Depending on the operative conditions, various rate laws describe the oxidation processes [20-22]. Usually, for thin films logarithmic or cubic functions relate layer thickness and oxidation time, but parabolic and linear dependencies are found for thicker layers. In the medium temperature range (700-1000°C) the kinetics of air oxidation of titanium is similar to that in oxygen [21], and the total absorption by titanium is comparable; at higher temperatures (1200°C) the oxidation rate in oxygen atmosphere is higher than in air [15]. The process is complicated by the fact that the metal reacts not only with oxygen but also with nitrogen, and formation of a defective ruffle lattice by inserting nitrogen facilitates diffusion and increases the oxidation rate [23]. Our results indicate the onset of X-ray detectable solid solubility of oxygen in the h.c.p, o:-Ti structure at ca 600°C reaching a value of d (002) = 2.380 A in the sample oxidized at 800°C for 60 min. This is in agreement with previous indications that the dissolution of oxygen in the metal lattice takes place at a fairly low rate below 700°C [10, 24] and sufficiently rapidly at 800°C to constitute an appreciable part of the total oxidation, even during the initial stages [12, 25, 26]. Indexing of the suboxide spectra on the basis of the titanium pattern leads to unit-cell dimensions a = 2.967 and c = 4.756 A in the 800°C (60 min) sample, which may be compared to a = 2.950 and c = 4.686 A for Ti (ASTM 5-0682) and a = 2.9593 and c = 4.8454 ~ for Ti20 [27]. As the c-axis expansion is particularly characteristic of the oxygen content, this suggests a phase with less dissolved oxygen in the octahedral interstices of the titanium lattice than in case of the maximum solubility limit at Ti20. The results at 800, 900 and 1300°C conform to Ti30 with subcell a = 2.969 and c---4.769A (supercell: a = 5.1418 and c = 14.308A, space group P312) [8]. The TiO0+x and TieO1 y structures essentially consist of a close-packed hexagonal arrangement of titanium atoms with some layers of octahedral interstices normal to the c-axis vacant. It is well known that oxygen dissolves in the octahedral interstices of the titanium lattice up to a concentration of 34 at. % [28-30]. In the limiting composition (Ti20), which is of the anti-Cd(OH)2 type, only half of the
772
V. ANTONUCCI, N. GIORDANO AND J. C. J. BART
available interstitial sites are occupied. The structure is stable at oxygen contents lower than the composition Ti20, indicating the existence of random vacancies in the oxygen layers. At the composition Ti30, which crystallizes as a Ti20 super-structure [8], one third of the oxygen sites in the occupied layers is empty. Although our experimental technique is probably not sufficient to detect weak super-lattice reflections, the preparative method (without annealing) is likely to lead to disordered suboxide structures, i.e. with a random rather than ordered arrangement of the oxygen atoms. The maximum content of oxygen (x or y) is dependent on the heat treatment. In previous work the oxygen content of 15 at. % in the outer layers of the titanium metal oxidized at 650-700°C has been ascribed to Ti60, with a further increase only after a relatively long period of oxidation [10]. We have found no indications for a particular thermodynamic stability of this stoichiometry under our experimental conditions. Rather, the presence of two (002) reflections in the 1000°C samples indicates the stability of various suboxides in the phase range Ti30-Ti20. However, this aspect was not further investigated. It is of interest to notice the formation of the suboxides TiO0+x and Ti2Ol-y together with TiO2 in our quite different preparative procedures. This is qualitatively in accordance with the detection of Ti20 at 700-882°C [31] and in high temperature oxidation of titanium (1000-1500°C) [25]. However, it is surprising that no other intermediate oxides (TiOx, 0.5 < x < 2) were detected, in contrast to other experiences [10, 15, 25, 31-33]. This may be due to the fact that we interrupted the oxidation process in the early stages (cf. Fig. 12 of ref. [15]). It is quite likely that intermediate oxides TiOx are present at the interface of the titanium core (with oxygen dissolved in the octahedral interstices) and the external rutile layer. However, our results suggest that these oxide concentrations are so small that they elude detection by bulk techniques such as X-ray diffraction. XPS could eventually be helpful here if the (thick) rutile layer could be removed by sputtering techniques without altering the system.
Photoefficiency and structure
As to the solid state properties which are responsible for the photoefficiency of our electrodes, we notice the appreciable rise in r/at 600°C for the samples oxidized for 45 and 60 min, in which TiO0+x is X-ray detected (together with rutile). Although at 700°C r/ initially increases with the amount of TiO0+x, the subsequent drop (at 60 min) is unrelated to the suboxide layer thickness, which increases. The findings at 800°C with drastically lower r/for higher oxidation times and the high suboxide layer thickness in the 45 min sample confirm this view. For similar reasons, a linear correlation between r~ and the rutile layer thickness is excluded. Actually, for the amounts of suboxide which
do not vary greatly in the 800°C series, the efficiency drops with the increase in the TiO2 layer thickness. Apparently, an optimal suboxide-rutile distribution is found in the 800°C sample oxidized for 15 min. As to the much better performance of the flame-oxidized electrodes, it should be observed that for low reaction times (up to 5 min) the total oxide layer thickness is less than that for the electrode oxidized in the furnace at 800°C for 15 min, as indicated by the intensity of the diffraction lines of the Ti substrate in the X-ray spectrum of the 1300°C samples. This suggests a role of the overall layer thickness. The pronounced drop in the efficiency after 2 min at 1300°C again corresponds to a steep increase in rutile formation. Essentially, therefore, a variety of factors are in play. This is also clearly seen by comparing the flame-oxidized sample (1 min) and the 800°C sample (45 min), which give quite different performances. The latter shows slightly thicker TiO2 and suboxide layers, whereas the former exhibits the X-ray detectable titanium substrate. On the whole, our results suggest that the presence of the suboxide at the metal-semiconductor interface is somehow related to the photoefficiency, although the overall performance of the electrode depends also on the thickness of the intermediate TiO0+x or Ti2OI-y layer and external TiO2 skin. If we consider that oxidation of titanium leads to uniform oxide layers parallel to the surface with a random distribution of porosity throughout the oxide scale, eventually together with whisker formation [10], it is important to recall that the typical penetration depth of ultraviolet light into TiO2 is only ca 2 #m [34]. Whereas for considerably thicker layers (at least 1015/~m), as in the samples 800°C (60 min) and 1300°C (20 min), the photoefficiency is low, it is difficult to establish whether the suboxide layer is photoirradiated in the most efficient electrodes. It should also be remembered that oxides such as Ti203 and TiO show typically near metallic conduction and that the conductivities of titanium metal samples with oxygen in solid solution are roughly of the same order of magnitude as those of the TiO-phase materials [35]. Therefore, it is not certain that the improvement in the photoelectrochemical efficiency of the conversion of solar into chemical energy over oxidized titanium is connected with the presence of dissolved oxygen atoms in the metal, even though a phenomenological relationship is suggested. The fact that /Tmaxshifts towards shorter oxidation times at higher temperatures (Fig. 1) may stand in relation to the perfection (instead of thickness) of the rutile layer. Although in our experimental conditions we have been unable to detect rutile-like structures [36--38], such as the discrete Magn61i phases [30, 39, 40], the presence of Ti 3÷ ions on the surface is necessary for the chemisorption of water [41]. As the oxidation rate increases proportionally more rapidly at higher temperatures, prolonged gas-solid interaction leads to thermodynamic equilibrium with the formation of stoichiometric TiO2. It is apparently quite important to avoid the thermodynamic equilibrium of reaction (1) and to
773
OXIDIZED TITANIUM ELECTRODES
Information regarding the oxidation mechanism is given in the Arrhenius plot (Fig. 2). Between 300 and 400°C, TiO2 (rutile and eventually anatase, the latter not observed in our conditions) is formed slowly. The influence of the formation of the lower valent titanium centres is only apparent at 700°-800°C and the intermediate temperature range is a transitional stage.
t300 %
0.7 ¸
\ % \
\ \
0.5
o~ • 700
Acknowledgement--Thanks are due to Mr. V. Bozzola for the X-ray measurements.
0'31 0.1
REFERENCES 15 '
3'0
4'5
l-
Fig. 1. Maximum photoelectrochemical efficiency (77%) for titanium oxidized between 600 and 800°C (in the furnace) and at 1300°C (in the flame) as a function of the oxidation time (min). interrupt the oxidation process at an earlier stage to preserve the lower valent titanium centres. Apart from the menovalent effects due to the cations in the substoichiometric layer, which positively influence the photoconductivity, we should also consider the greater basicity of the surface. Namely, reduction of TiOz leads to an increase in IEPS (isoelectric point of solids) of the surface of the solid (6.0 for TiO2, 7.4 after H2 reduction for 10 min) [42]. This variation affects the photoconductivity by lowering the band-gap from 3.0 to 2.4eV (V. Antonucci, unpublished results). The influence of IEPS is also corroborated by our results on doping of TiO2 in which dopant oxides of higher IEPS improve the photoelectrochemical efficiency of the electrode (V. Antonucci, unpublished results).
,j300 ",-.,~8o0
gv
\
w.O-
g %% %% %%
2.0
I
I
'
I
I
2
3
I/T
Fig. 2. Arrhenius plot relating the maximum photoelectrochemical efficiency and oxidation temperature. , Various rate laws; . . . . , transitional stage.
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27. S. Andersson, B. Coll6n, G. Kruuse, U. Kuylenstierna, A. Magn61i, H. Pestmalis and S. Asbrink, Acta chem. scand. 11, 1653 (1957). 28. E. S. Bumps, H. D. Kessler and M. Hansen, Trans. Am. Soc. Metals 45, 1008 (1953). 29. W. Rostoker, J. Metals 4, 981 (1952). 30. S. Andersson, B. Coll6n, U. Kuylenstierna and A. Magneli, Acta chem. scand. 11, 1641 (1957). 31. D. Simon, C. Perrin and J. Bardolle, C. r. hebd. S~anc. Acad. Sci., Paris 283C, 299 (1976). 32. O. Matsumoto, Denki Kagaku 38, 826 (1970); Chem. Abstr. 74, 130891e (1971). 33. G. P. Luchkin, Issled zharopr. Splay. Akad. Nauk. SSSR, Inst. Met. im A. A. Baikova 7,227 (1961); Chem. Abstr. 56, 8427h (1962).
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0360-3199/82/1007694)6 $03.00/0 Pergamon Press Ltd. 1982 International Association for Hydrogen Energy.
S T R U C T U R E A N D P H O T O E L E C T R O C H E M I C A L EFFICIENCY OF O X I D I Z E D TITANIUM E L E C T R O D E S V.
ANTONUCCI,*
N. GIORDANO* and J. C. J.
BARTt
*Istituto C. N. R. di Ricerche sui Metodi e Processi Chimici per la Trasformazione e l'Accumulo dell'Energia, Pistunina, Messina, Italy; tIstituto G. Donegani, Via G. Fauser 4, 28100 Novara, Italy
(Received for publication 29 December 1981) Abstraet--Pure titanium laminae were oxidized in air between 300 and 1300°C and used as electrodes in the photodecomposition of water. The maximum photoelectrochemical conversion was found for flame-oxidized (1300°C) samples. Although the efficiency shows a parallelism with the presence of the metallic interstitial compounds TiO0+x (x < 0.33) and Ti20~-y (0.33 > y > 0) at the metal-semiconductor interface, the thickness of the suboxide layer and that of the external rutile scale, it is argued that other factors (optimal suboxide-rutile distribution, overall layer thickness, layer perfection, non-stoichiometry of TiO2, isoelectric point of solids (IEPS), etc.) may be more important in the process.
(anatase, brookite, h. p. form), TiO (ASTM 23-1078) and Ti,O2,- 1 (where n = an integer) were not detected. In order to relate the structure and efficiency of photoelectric conversion, the electrodes (0.5 cm) were immersed in a homogeneous photoelectrochemical cell containing a liquid electrolyte (0.1 N Na2SO4), a Pt counter-electrode (Engelhard, 15 cm2), a current meter (Keithley Multimeter) and an external bias device. A 400W mercury lamp (Osram), emitting in the 3101000nm range, was used as the light source. The efficiency
INTRODUCTION Titanium dioxide is widely used in the photosensitized electrolysis of water because of its non-corrosive properties, compared to other semiconductors, in spite of the high band-gap (3.02 eV) which limits its efficiency in the conversion of irradiated visible light into chemical energy. TiO2 is often used in single crystal form [1], or more recently also as polycrystalline material obtained from titanium compounds [2], deposition or evaporation as thin films [3] and electrochemical [4] or thermal oxidation of metallic sheets [5]. As very little [6] has been reported on the influence of the preparation of such devices on their energy balance and structure, in this paper we study the thermal oxidation of metallic laminae in relation to their performance as photoelectrodes.
r/= Ic(1.23 - V) × lO0/Wa was calculated according to Ghosh and Maruska [9], where Ic is the cell current (mA cm-2), V the bias and W, the energy content of incident light (measured with an electrosolarimeter, Sun System F D R 5003 B), in conditions with (0.2 and 0.5 V) and without bias.
EXPERIMENTAL As the oxidation kinetics of titanium and titaniumbased alloys is influenced by a variety of factors, for example the nature of the gas phase, the surface treatment of the metal and its chemical composition [7], pure titanium laminae (VT1, 99.5% Ti; principal impurities Fe, Si, C; 0.5 mm thick) were oxidized in controlled conditions, namely in air without pretreatment of the surface. The material was exposed to air in an electric furnace between 300 and 1000°C for various lengths of time (from 15 min up to i h in 15 rain intervals). A second series of electrodes was flameoxidized at 1300°C for 1-20 min. Phase analysis was carried out at room temperature in reflectance geometry by directly mounting the electrodes on a carefully calibrated X-ray powder diffractometer (Philips) using CuKo:radiation (). = 1.5418/~). The spectra were interpreted with reference to rutile (ASTM 21-1276), Ti (ASTM 5-0682), Ti20 (ASTM 11-218) and Ti30 (ref. [8]); other polymorphs of TiO:
RESULTS No new (bulk) phase formation in the electrodes was detected by X-ray diffraction methods up to 500°C (see Table 1). Small amounts of rutile were observed after oxidation in air at 600°C for 15 min; the TiO2 layer thickness increased with reaction time. Rutile formation conforms with previous results [10--13]. In the sample oxidized at 600°C for 60min, a titanium suboxide (TiO0+x, x < 0.33) [14] was detected by line-broadening at the low angle side of the oriented Ti (002) reflection. At 700°C the effects were more pronounced with increased formation of the two oxides; after activation for 60 min at this temperature line-broadening was also detected at the weak Ti (012) reflection. At 800°C the characteristic reflections of metallic titanium strongly decreased in intensity, especially after prolonged oxidation (1 h), thus indicating the presence of a fairly thick oxide layer (estimated thickness 10-15 #m). The non-oriented rutile scale grew rapidly at this tempera-
769
V. ANTONUCCI, N. GIORDANO AND J. C. J. BART
770
Table 1. Phase analysis of oxidized titanium electrodes* Temperature (°C) 300-500 600 700 800 900
1000 1300
Time (min)
Ti
15-60 1545 60 15-60 15-30 45 60 155 60~: 15§ 30-60§ 15-45:~§ 605 1-2 3-5 20
O O O O + + O + t
Phase distribution TiO0÷~ Ti30 + + + + + + t
+ + + + + + + + +
Ti2Ol-y
TiO2
+ + ++ ++ -
+t + + + + + + + + + + + O + + +
* Results of XRD, unless otherwise indicated. 5 Surface structure (XPS). ~: Before flaking; o, epitaxial orientation. § After flaking. t, traces; +, present; - , absent.
ture and TiO2 was the main phase after 1 h at 800°C when metallic titanium was completely X-ray shielded. A t increasing reaction times at 800°C the diffraction bands of TiO0 +x gradually shifted away from the angular positions of metallic titanium. The maximum suboxide layer thickness was reached after 45--60 min at 800°C. In the 900°C samples the average suboxide (002) position is 37.70 ° (d = 2.386 ]~), which is to be compared to those of Ti (38.430°), Ti30 (37.730 °) and Ti20 (37.123°). Oxidation at 900°C leads to poor adherence of the oxide surface layers to the grey metal core (cf. also ref. [15]). This indicates considerable stress at the interface, probably the result of the volume change accompanying the conversion of metal to oxide [10, 16]. Before the white surface scale flakes off the core, the X-ray spectra of the 900 ° samples show mainly futile together with some Ti30. Afterwards, the layer consists of a thin residue of TiO2 on top of a thick Ti30 substrate. The titanium core was never brought out in the spectra of the 900°C samples. A t higher oxidation times (up to 1 h) the increasing (002) line asymmetry indicates the presence of a second suboxide (Ti2Ol-y with y < 0.33 or a Ti20 phase with an oxygen deficit) in minor amounts at c a 37.35 ° ( d = 2.408A). A f t e r deconvolution the gradual narrowing of the (002) reflection with activation time (hla values of 0.40 ° and 0.30 ° after 15 and 60 min, respectively) indicates either an increase in crystallite size or a more h o m o g e n e o u s oxygen dissolution. Essentially similar results were found at 1000°C. These electrodes have a white-grey aspect, indicating a fair degree of adherence of the (residual) rutile layer to the substrate. Two Ti2Ol-y phases were clearly distinguishable by the (002) reflections at 37.59 ° and 37.28 °
after oxidation for 15 min, and at 37.45 ° and 37.24 ° after 60 rain. The flame-oxidized samples (1300°C) are in line with the previous picture and show increasing amounts of TiO2 (rutile) on a Ti substrate (detectable in the X-ray spectra) in the presence of substantial quantities of TiO0+x(x < 0.33) and Ti30. The Ti30 layer thickness is almost constant during the initial oxidation period (1-4 min) and is slightly inferior to that in the sample oxidized at 800°C for 45 min. A f t e r prolonged heating the suboxide layer thickness decreased strongly and a thick oriented rutile layer was observed instead. The Ti30 crystallite size (measured for the (002) plane) increased from c a 160 to 300/~ during the oxidation process (1-20 min). No Ti reflections were observed in the X-ray spectrum of the sample flame-oxidized for 20 min, as opposed to TiO2, Ti30 and a small residue of TiO0+x as a shoulder of the Ti30 (002) reflection at the high angle side. The slight, but significant, difference in the suboxide (002) lattice position before and after loss of the rutile surface layer at 900°C (from 37.69 ° to 37.74 °, 20) indicates the variable composition of the scale, which is tess oxygen-rich at greater depth, in accordance with the expectation. A t higher temperatures (cf. 1000 ° and 900°C) m o r e oxygen is dissolved, as expressed by the shift of the (002) reflection to lower 20 values, and one of the Ti201 -y phases approaches the Ti20 composition. The photoelectrochemical efficiency (r/%) for titanium electrodes oxidized between 300 and 1000°C in the furnace and those flame-oxidized at 1300°C are given in Tables 2 and 3, respectively, for various values of the external bias. U p to 800°C the most efficient
771
OXIDIZED TITANIUM ELECTRODES Table 2. Photoelectrochemical efficiency (~/%) for titanium electrodes oxidized between 300 and 800°C at various values of the external bias t=15'
%
t=30'
%
T (°C)
V= 0
0.2
0.5
0
0.2
300 400 500 600 700 800
0.01 0.015 0.064 0.083 0.241 0.317
0.01 0.025 0.068 0.084 0.227 0.309
0.017 0.028 0.067 0.073 0.176 0.268
0.013 0.024 0.088 0.098 0.275 0.076
0.014 0.039 0.091 0.107 0.262 0.082
t=45'
0.5 0.023 0.045 0.101 0.105 0.217 0.080
electrodes were those oxidized at higher temperatures for gradually lower reaction times. The maximum efficiency was found for a flame-oxidized (1300°C) sample, in accordance with the literature [6]. Obviously, under these conditions the formation of a non-stoichiometric titanium oxide layer with good semiconducting properties requires very short oxidation times; at prolonged exposure to air the stoichiometric compound TiO2 is formed, which is quite unfavourable for photoelectrolysis. Essentially similar observations apply to the 300-800°C range. Due to flaking, application of the laminae oxidized at 900-1000°C in the photoelectrochemical cell was not considered. DISCUSSION
Physico-chemical characterization Thermodynamic considerations indicate that TiO2 is the most probable crystalline product of titanium oxidation under a variety of conditions of gaseous atmosphere (air, steam, CO2), temperature and time, according to the irreversible processes Ti(s) + O2(g)
--+ TiO2(s)
(1)
Ti (s) + 2H20 (g) --+ TiO2 (s) + 2H2 (g)
(2)
Ti (s) + 2CO2 (g) -+ TiO2 (s) + 2CO (g).
(3)
However, in spite of a fair degree of agreement in Table 3. Photoelectrochemical efficiency (r/%) for titanium electrodes flame-oxidized at 1300°C at various values of the external bias
r (oc) 130o t 1' 1' 30" 2' 3' 4' 5' 10' 20'
%
V= 0
0.2
0.5
0.7
0.9
0.45 0.49 0.74 0.57 0.37 0.33 0.32 0.15
0.45 0.51 0.65 0.52 0.38 0.31 0.31 0.14
0.39 0.46 0.51 0.44 0.34 0.27 0.27 0.12
0.33 0.39 0.46 0.42 0.39 0.23 0.23 0.10
0.33 0.30 0.36 0.37 0.30 0.18 0.18 0.10
%
0
0.2
0.022 0.027 0.039 0.130 0.300 0.074
0.033 0.037 0.049 0.144 0.295 0.095
t=60'
0.5 0.055 0.041 0.048 0.114 0.255 0.100
%
0
0.2
0.5
0.015 0.032 0.032 0.172 0.177 0.044
0.027 [).033 0.035 0.175 0.189 0.045
0.031 0.041 0.034 0.174 0.178 0.044
experimental oxidation rates, phase compositions and proposed oxidation mechanisms differ widely [17-19]. Depending on the operative conditions, various rate laws describe the oxidation processes [20-22]. Usually, for thin films logarithmic or cubic functions relate layer thickness and oxidation time, but parabolic and linear dependencies are found for thicker layers. In the medium temperature range (700-1000°C) the kinetics of air oxidation of titanium is similar to that in oxygen [21], and the total absorption by titanium is comparable; at higher temperatures (1200°C) the oxidation rate in oxygen atmosphere is higher than in air [15]. The process is complicated by the fact that the metal reacts not only with oxygen but also with nitrogen, and formation of a defective ruffle lattice by inserting nitrogen facilitates diffusion and increases the oxidation rate [23]. Our results indicate the onset of X-ray detectable solid solubility of oxygen in the h.c.p, o:-Ti structure at ca 600°C reaching a value of d (002) = 2.380 A in the sample oxidized at 800°C for 60 min. This is in agreement with previous indications that the dissolution of oxygen in the metal lattice takes place at a fairly low rate below 700°C [10, 24] and sufficiently rapidly at 800°C to constitute an appreciable part of the total oxidation, even during the initial stages [12, 25, 26]. Indexing of the suboxide spectra on the basis of the titanium pattern leads to unit-cell dimensions a = 2.967 and c = 4.756 A in the 800°C (60 min) sample, which may be compared to a = 2.950 and c = 4.686 A for Ti (ASTM 5-0682) and a = 2.9593 and c = 4.8454 ~ for Ti20 [27]. As the c-axis expansion is particularly characteristic of the oxygen content, this suggests a phase with less dissolved oxygen in the octahedral interstices of the titanium lattice than in case of the maximum solubility limit at Ti20. The results at 800, 900 and 1300°C conform to Ti30 with subcell a = 2.969 and c---4.769A (supercell: a = 5.1418 and c = 14.308A, space group P312) [8]. The TiO0+x and TieO1 y structures essentially consist of a close-packed hexagonal arrangement of titanium atoms with some layers of octahedral interstices normal to the c-axis vacant. It is well known that oxygen dissolves in the octahedral interstices of the titanium lattice up to a concentration of 34 at. % [28-30]. In the limiting composition (Ti20), which is of the anti-Cd(OH)2 type, only half of the
772
V. ANTONUCCI, N. GIORDANO AND J. C. J. BART
available interstitial sites are occupied. The structure is stable at oxygen contents lower than the composition Ti20, indicating the existence of random vacancies in the oxygen layers. At the composition Ti30, which crystallizes as a Ti20 super-structure [8], one third of the oxygen sites in the occupied layers is empty. Although our experimental technique is probably not sufficient to detect weak super-lattice reflections, the preparative method (without annealing) is likely to lead to disordered suboxide structures, i.e. with a random rather than ordered arrangement of the oxygen atoms. The maximum content of oxygen (x or y) is dependent on the heat treatment. In previous work the oxygen content of 15 at. % in the outer layers of the titanium metal oxidized at 650-700°C has been ascribed to Ti60, with a further increase only after a relatively long period of oxidation [10]. We have found no indications for a particular thermodynamic stability of this stoichiometry under our experimental conditions. Rather, the presence of two (002) reflections in the 1000°C samples indicates the stability of various suboxides in the phase range Ti30-Ti20. However, this aspect was not further investigated. It is of interest to notice the formation of the suboxides TiO0+x and Ti2Ol-y together with TiO2 in our quite different preparative procedures. This is qualitatively in accordance with the detection of Ti20 at 700-882°C [31] and in high temperature oxidation of titanium (1000-1500°C) [25]. However, it is surprising that no other intermediate oxides (TiOx, 0.5 < x < 2) were detected, in contrast to other experiences [10, 15, 25, 31-33]. This may be due to the fact that we interrupted the oxidation process in the early stages (cf. Fig. 12 of ref. [15]). It is quite likely that intermediate oxides TiOx are present at the interface of the titanium core (with oxygen dissolved in the octahedral interstices) and the external rutile layer. However, our results suggest that these oxide concentrations are so small that they elude detection by bulk techniques such as X-ray diffraction. XPS could eventually be helpful here if the (thick) rutile layer could be removed by sputtering techniques without altering the system.
Photoefficiency and structure
As to the solid state properties which are responsible for the photoefficiency of our electrodes, we notice the appreciable rise in r/at 600°C for the samples oxidized for 45 and 60 min, in which TiO0+x is X-ray detected (together with rutile). Although at 700°C r/ initially increases with the amount of TiO0+x, the subsequent drop (at 60 min) is unrelated to the suboxide layer thickness, which increases. The findings at 800°C with drastically lower r/for higher oxidation times and the high suboxide layer thickness in the 45 min sample confirm this view. For similar reasons, a linear correlation between r~ and the rutile layer thickness is excluded. Actually, for the amounts of suboxide which
do not vary greatly in the 800°C series, the efficiency drops with the increase in the TiO2 layer thickness. Apparently, an optimal suboxide-rutile distribution is found in the 800°C sample oxidized for 15 min. As to the much better performance of the flame-oxidized electrodes, it should be observed that for low reaction times (up to 5 min) the total oxide layer thickness is less than that for the electrode oxidized in the furnace at 800°C for 15 min, as indicated by the intensity of the diffraction lines of the Ti substrate in the X-ray spectrum of the 1300°C samples. This suggests a role of the overall layer thickness. The pronounced drop in the efficiency after 2 min at 1300°C again corresponds to a steep increase in rutile formation. Essentially, therefore, a variety of factors are in play. This is also clearly seen by comparing the flame-oxidized sample (1 min) and the 800°C sample (45 min), which give quite different performances. The latter shows slightly thicker TiO2 and suboxide layers, whereas the former exhibits the X-ray detectable titanium substrate. On the whole, our results suggest that the presence of the suboxide at the metal-semiconductor interface is somehow related to the photoefficiency, although the overall performance of the electrode depends also on the thickness of the intermediate TiO0+x or Ti2OI-y layer and external TiO2 skin. If we consider that oxidation of titanium leads to uniform oxide layers parallel to the surface with a random distribution of porosity throughout the oxide scale, eventually together with whisker formation [10], it is important to recall that the typical penetration depth of ultraviolet light into TiO2 is only ca 2 #m [34]. Whereas for considerably thicker layers (at least 1015/~m), as in the samples 800°C (60 min) and 1300°C (20 min), the photoefficiency is low, it is difficult to establish whether the suboxide layer is photoirradiated in the most efficient electrodes. It should also be remembered that oxides such as Ti203 and TiO show typically near metallic conduction and that the conductivities of titanium metal samples with oxygen in solid solution are roughly of the same order of magnitude as those of the TiO-phase materials [35]. Therefore, it is not certain that the improvement in the photoelectrochemical efficiency of the conversion of solar into chemical energy over oxidized titanium is connected with the presence of dissolved oxygen atoms in the metal, even though a phenomenological relationship is suggested. The fact that /Tmaxshifts towards shorter oxidation times at higher temperatures (Fig. 1) may stand in relation to the perfection (instead of thickness) of the rutile layer. Although in our experimental conditions we have been unable to detect rutile-like structures [36--38], such as the discrete Magn61i phases [30, 39, 40], the presence of Ti 3÷ ions on the surface is necessary for the chemisorption of water [41]. As the oxidation rate increases proportionally more rapidly at higher temperatures, prolonged gas-solid interaction leads to thermodynamic equilibrium with the formation of stoichiometric TiO2. It is apparently quite important to avoid the thermodynamic equilibrium of reaction (1) and to
773
OXIDIZED TITANIUM ELECTRODES
Information regarding the oxidation mechanism is given in the Arrhenius plot (Fig. 2). Between 300 and 400°C, TiO2 (rutile and eventually anatase, the latter not observed in our conditions) is formed slowly. The influence of the formation of the lower valent titanium centres is only apparent at 700°-800°C and the intermediate temperature range is a transitional stage.
t300 %
0.7 ¸
\ % \
\ \
0.5
o~ • 700
Acknowledgement--Thanks are due to Mr. V. Bozzola for the X-ray measurements.
0'31 0.1
REFERENCES 15 '
3'0
4'5
l-
Fig. 1. Maximum photoelectrochemical efficiency (77%) for titanium oxidized between 600 and 800°C (in the furnace) and at 1300°C (in the flame) as a function of the oxidation time (min). interrupt the oxidation process at an earlier stage to preserve the lower valent titanium centres. Apart from the menovalent effects due to the cations in the substoichiometric layer, which positively influence the photoconductivity, we should also consider the greater basicity of the surface. Namely, reduction of TiOz leads to an increase in IEPS (isoelectric point of solids) of the surface of the solid (6.0 for TiO2, 7.4 after H2 reduction for 10 min) [42]. This variation affects the photoconductivity by lowering the band-gap from 3.0 to 2.4eV (V. Antonucci, unpublished results). The influence of IEPS is also corroborated by our results on doping of TiO2 in which dopant oxides of higher IEPS improve the photoelectrochemical efficiency of the electrode (V. Antonucci, unpublished results).
,j300 ",-.,~8o0
gv
\
w.O-
g %% %% %%
2.0
I
I
'
I
I
2
3
I/T
Fig. 2. Arrhenius plot relating the maximum photoelectrochemical efficiency and oxidation temperature. , Various rate laws; . . . . , transitional stage.
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