Photoelectrochemical study of (Ti,V)O2 and (Ti,Nb)O2 alloys

Solar Energy Materials 9 (1983) 101-111 North-Holland

PHOTOELECTROCHEMICAL ALLOYS

101

STUDY OF (Ti,V)O 2 AND (Ti,Nb)O 2

Jacques G A U T R O N , Philippe L E M A S S O N * Laboratoire d'Electrochimie lnterfaciale du CNRS, 1, Place A Briand, 92195 Meudon Cedex, France

Bertrand P O U M E L L E C a n d J e a n - F r a n c i s M A R U C C O Laboratoire des Compos~s Non-Stoechiom~triques ERA 680 du CNRS, Universit~ de Paris Sud, 91405 Orsay Cedex, France

Received 8 September 1982; in revised form 4 November 1982; in final form 15 February 1983 The ceramic alloys (Ti,Nb)O2 and (Ti,V)O2 are studied photoelectrochemically in an aqueous electrolyte. A n-type electrical conductivity is noted for the whole composition range. The energy gaps Eg for the (Ti,V)O2 alloys are 0.8 eV (VO2) < Eg < 3 eV (TiO2) whereas for the (Ti,Nb)O2 alloys Eg remains nearly constant and ~ 3 eV. With an energy gap of ---2 eV, the alloy Ti 04V0.602, which is electrochemicallystable, seems especially attractive for solar energy conversion. However, its characteristics are counterbalanced by a too much anodic flat-band potential and by a low quantum efficiency. A correlation between the photocurrents and the associated optical transitions leads to precise band diagrams of both alloys. The study of their photoelectrochemical stability enables one to determine the nature of the optical transitions, giving rise to the photocurrent.

1. Introduction

The p r o b l e m of s e m i c o n d u c t o r d e c o m p o s i t i o n is of m a i n i m p o r t a n c e in the d e v e l o p m e n t of liquid j u n c t i o n photocells. U p to now, it drastically b u r d e n s the use as of classical s e m i c o n d u c t o r electrodes with adequate energy gap (ranging b e t w e e n 1 a n d 2 eV like Si, G a A s , CdTe, InP, CdSe . . . . ), for o p t i m u m conversion efficiency. D u e to its high electrochemical stability, t i t a n i u m dioxide has b e e n extensively studied as a model electrode, d u r i n g the last decade, despite a wide gap ( = 3 eV) which makes it weakly attractive for actual use. The action spectrum of TiO2 is oxygen deficiency (TIO2_8) [la] as well as sensitive to d o p i n g by Cr, A1, M n . . . . [2]. However, the spectral b r o a d e n i n g towards the visible is m o d e r a t e a n d conversion efficiency at energies lower than 3 eV r e m a i n s weak. Such a p h e n o m e n o n is c o n n e c t e d with the fact that reduction a n d / o r d o p i n g of T i O 2 do n o t p r o d u c e new energy b a n d s b u t discrete energy levels localized in the f o r b i d d e n gap. The optical transitions associated with these levels c o r r e s p o n d to low

* To whom correspondence should be addressed. 0 1 6 5 - 1 6 3 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d

102

J. Gautron et al. / ('eramic (7i, I')O: and ( Tz. Nb)O: alh~v~

values of the optical a b s o r p t i o n coefficient and the generated p h o t o c a r r i e r s generall5, have a low m o b i l i t y in them. A m o n g the possibilities for the m o d i f i c a t i o n of the energy gap of TiO 2. one is to m a k e a new m a t e r i a l by alloying it with a n o t h e r oxide XO 2 exhibiting crystal and electronic structures c o m p a t i b l e with that of T i O 2. In this work, two different X O 2 oxides were used: N b O 2 a n d VO 2 which give solid solutions (Ti,X)O 2 in the whole c o m p o s i t i o n range [3a,b]. At r o o m temperature, these oxides have an energy gap lower than 1 eV [4]. Therefore it seems r e a s o n a b l e to i m a g i n e that the energy gaps of the alloys will range between that of XO~ and of T i O 2 like those of the classical t e r n a r y alloys of I I I - V or I I - V I semiconductors. W e show in fig. 1 the energy b a n d d i a g r a m s of the three oxides as the5 are r e p o r t e d in the literature [4 6]. W e p o i n t out two features c o n c e r n i n g the diagrams: a) The energy of the top of the ,rr b a n d is a s s u m e d to stay invariant for the three oxides. The reason for this is that in each case, this b a n d is mainly c o n s t i t u t e d by the p - o r b i t a l s of oxygen. b) Energy b a n d s are related with the d-levels in the metal. A m o n g them, t!l which c o r r e s p o n d s to c a t i o n - c a t i o n i n t e r a c t i o n s parallel to the c axis in the rutile structure. This greatly influences the physical p r o p e r t i e s of the oxides. In TiO 2, tlj is not distinct of the o t h e r d levels which constitute the c o n d u c t i o n b a n d ,~*: the d i s t a n c e of 3 eV between ,~ a n d ~r * is the energy gap of this

I iI

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!

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% ~p~ ,,, s 0 z-

TiOz

VOz

N bOz

Fig. 1. Energy diagrams of TiO2. VO2 from ref. [4] and NbO2 from ref. [6]. The gaps are indicated approximately. For the three oxides, the position of the ~ band has been assumed to be constant.

J. Gautron et al. / Ceramic (Ti, V)O2 and (Ti, Nb)O 2 alloys

103

semiconductor. On the contrary, for VO 2 and N b O 2, tll is distinct of ~ * and splits into two energy bands which are the conduction and the valence band. The corresponding energy gap is lower than 1 eV. Then, it is expected that VO 2, N b O 2 and the alloys (Ti,V)O 2 and (Ti,Nb)O 2 present a photoelectrochemical behaviour completely different of that of TiO 2. In the following, we show experimentally that these assumptions are verified only for the (Ti,V)O 2 alloys. From this discrepancy between the observed photocurrents and the energy diagrams of the literature, the interest of photoelectrochemistry in the investigation of solid state properties of new materials is pointed out.

2. Experimental The samples are sintered from powders of the respective oxides whose purity is at least 99.99%. 2.1. (Ti, V)O_, alloys

VO 2 is prepared from a mixture V203 + V205 which is vacuum sealed in a quartz ampoule and heated at 700°C for 3 days. The final product is characterized by X-ray spectroscopy. The solid solutions with TiO 2 are elaborated from titanium and vanadium dioxide powders in required proportions. The samples are pressed as pellets, and heated slowly up to 1200°C, which is maintained for 48 h to achieve the sintering. The resulting materials are well-defined [3a]. At room temperature, in the whole range of composition, the electric conductivity is about 10 -2 ~2-J c m - i 2.2. (Ti, Nb)O_, alloys

The samples are sintered from a mixture of TiO 2 and Nb205 powders, pressed and heated for 24 h at 1400°C in argon atmosphere. These ceramics are then reduced at 1100°C for 48 h in an (H2//O2) atmosphere, corresponding to an oxygen partial pressure in equilibrium with solid solutions (Ti,Nb)O 2 [7]. This reduction is controlled by thermogravimetry and solid solutions are formed according to Sakata [3b] and verified with our samples [8]. The electric conductivity of these alloys diminishes when [Nb]/[Ti] increases. The electrical conductivity is about l0 -4 f]-1 c m - i for N b O 2. The photoelectrochemical set up as well as the mounting of the electrodes are standard and described elsewhere [la]. The electrolyte is a 1 M K O H solution in purified water (pH = 14) and the potentials are referred to a m e r c u r y - m e r c u r o u s sulphate electrode (MSE). The ohmic contacts at the back-side of the oxide electrodes are made by ultrasonic soldering of an indium dot.

J. Gautron (,t aL ,. ( eramic (7i V)O: and (7~.,Vh)O, allm ,

104

3. Results 3. l. C u r r e n t

versus potential

characteristics

The d a r k current, p h o t o c u r r e n t and electrical p o t e n t i a l are d e n o t e d by i a, iph and V, respectively.

In fig. 2 are r e p o r t e d i a and iph versus V characteristics for a set of alloys. The5 are r e p r e s e n t a t i v e of the whole set of results we have o b t a i n e d . Their shapes are classical for n - t y p e s e m i c o n d u c t o r electrodes. The p o t e n t i a l value at which the onset of p h o t o c u r r e n t takes place (l{, n ) is of p a r t i c u l a r i m p o r t a n c e , since it enables us to position the energy b a n d s of our s e m i c o n d u c t o r s on the energy scale of the reference. V is used for this p u r p o s e instead of Vfb ( f l a t - b a n d p o t e n t i a l ) because of the n o n - l i n e a r i t y of the Schottky plots except in the case of T i O 2. G e n e r a l l y , V,n is m o r e a n o d i c than I/~.b [lb]. This arises from the high r e c o m b i n a t i o n rate for the p h o t o c a r r i e r s in the p o t e n t i a l range near Vrb. By increasing the p h o t o n flux, the difference (lJ}h-1..'i, . ) diminishes. In the p r e s e n t work, for T i O 2 with a 900 W t u n g s t e n - h a l o g e n l a m p as the light source, V,,, = - 1.8 V / M S E and Vfb = - 2 . 0 V / M S E . The difference 11~].b- I~i,,11is greater for the alloys than for T i O 2. This is due to the m o r e difficult sintering. Also. the s a t u r a t i o n value of iph is reached for greater b a n d - b e n d i n g than with TiO: (fig. 2). In table 1 are r e p o r t e d the values of V,,n for o u r various different samples. F o r the Ti~ V , O 2 ( x = 0 - 1 . 0 ) alloys, lJ~,,, is more a n o d i c than for TiO~. in good a g r e e m e n t with the results of Phillips et al. [9]. However, these authors indicate that P~,., = - 0 . 3 V / M S E as soon as x = 0.10 a n d then r e m a i n s nearly' c o n s t a n t for x increasing up to 0.50, in c o n t r a d i c t i o n with our results which indicate an a n o d i c v a r i a t i o n of Von for 0.40 _< x _< 0.60. It is worthwhile to m e n t i o n that the variation of V,,o seems to be n o n - m o n o t o n i c , with ¢~,~ (VO 2) being more c a t h o d i c than ~i,,~ (Ti0.aV0.60,,). F o r the Ti~_ ~Nb~O 2 ( x = 0 - 1 . 0 ) alloys: the small variations o b s e r v e d on ~'~ can certainly be a t t r i b u t e d to the r e c o m b i n a t i o n s of the p h o t o c a r r i e r s a n d we can assume that Vfb r e m a i n s nearly constant.

Table 1 Collected values of the photocurrent onset potential for the various allo)s x Von (V/MSE) Eg (eV) Ti I

0

0.40

0.45

-0.8 2.22

-0.5 2.14

0.04

0.20

0.50

0.80

1.0

- 1.3 2.78

- 1.5 2.76

1.2 2.91

- 1.3 = 2.95

1.2 2.99

- 1.8 2.98

0.60 -0.1 2.04

1.0 -2.1 < 1.2

~Nb~O_,

x K,° (V/MSE) Eg (eV)

0 -- 1.8 2.98

J. Gautron et al. / Ceramic (Ti, V)O, and (Ti, Nb)O 2 alloys

105

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Fig. 2. Current versus potential characteristics of TiO 2, Ti0.55V0.4502 and Ti0.50Nb0.5002 in 1 M KOH electrolyte (pH = 14). Note the position of Von in the three cases. Except for TiO 2, the saturation photo current is reached at extremely anodic potentials.

3.2. A ction spectra

The characteristics of iph versus photon energy (or wavelength k) at fixed potential correspond to optical interband transitions. In our case their measurement

106

J. Gautron et al. / ('eramw (ft, LQO, and (Ti, Nh)O! allov.~

enables determination of the energy gap for each sample. For this purpose, v,e use the Gartner model [10]; its adequacy in photoelectrochemistry we have previously demonstrated [11]. If we denote a as the optical absorption coefficient, W the width of the space charge region and L the minority carrier diffusion length, according to this model the fulfillment of conditions a W and c~L << 1 leads to iph C( (~. These conditions correspond to absorption at the band edge and adequately fit indirect transitions. An approximate equation relates c~, the photon energy h~, and the energy gap E~

oc (h,, - eg),,/2 along with n = 1, direct transition and n = 4, indirect transition. By plotting lp h.2/,, versus V it is possible to determine n for a linear relation (type of transition) and Eg. Experimentally, for convenient interpretation, iph m u s t be corrected from the response of the system. It is denoted by Nph. Results obtained with Ti] _,V,O2 are reported in fig. 3. When x increases, the spectrum shifts towards the visible ( E g - - 2 . 0 4 eV for x = 0.60). In the case x = 1 (VO2), the spectrum covers the complete experimental energy range (down to 1.2 eV); this agrees with the Eg value reported in fig. 1. These alloys have low efficiency, not only in the visible, but also in the UV range in contrast with TiO~. With Ti I - xNbxO2, the results are more surprising (fig. 4). The spectra iph v e r s u s

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/ Ceramic (Ti, V)O 2 and (Ti, Nb)O 2 alloys

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Fig. 3. Action spectra of Ti I_xVxO2 alloys. The potential is + 1 V / M S E . (a) Corrected photocurrent Nph vs. )L In the insert are reported the NpI~2 versus h~, plots which lead to the determination of Eg. (b) Quantum efficiency v/vs. %.

indicate a slight variation as c o m p a r e d to that of TiO 2. The result of increasing x is first to induce a slight shift towards the visible which is m a x i m u m for x = 0.20 (Eg -- 2.76 eV). Thereafter, the spectra are similar to that of TiO 2. F o r x = 1 ( N b O 2) we find E~ --- 2.99 eV, a value comparable to that obtained by Mavroides et al. [13]. However, in the latter case we notice the existence of a weak photocurrent at p h o t o n energies lower than 2.99 eV. F r o m the Npth/2 versus h~, plot (insert in fig. 4), a lower energy gap value could be deduced but the weak photocurrent cannot be attributed u n a m b i g u o u s l y either to a true interband transition or to an absorption tail related to impurities. In the former case, at least one of the b a n d s would be narrow leading to a low mobility of the charge carriers.

J. Gautron eta/. / Cerarntc (TL V)O, and (T~.Nb)O e alloL~

108

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400

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. . . . I . . . . PHOTONENERGY[eV]

Tio'8Nb0202

500 WAVELENGTH [nm]

Fig. 4. Uncorrected action spectra of T i ~ _ ~ N b x O 2 alloys. The potential is + 1 V / M S E . N o t e that a weak photocurrent is detectable with N b O 2 at wavelengths over 440 nm. In the insert is indicated the determination of Eg.

4. Discussion The oxides V O 2 and NbO 2 are semiconducting at room temperature and exhibit a semiconductor to metal transition at 340 K [14] and 1070 K [15], respectively. Such knowledge does not exist for the solid solutions Ti~ ,V,O 2 and Ti~ ,Nb,O 2. This arises from the difficulty interpreting the type of conductivity in these alloys, using classical measurements [16,17]. For this purpose, electrochemistry may be useful and in our case, an n-type conductivity is deduced from the current versus potential characteristics in the dark and under illumination. In the definition of a semiconductor, two parameters are especially important: the Fermi level position and the energy gap. These parameters can be determined by photoelectrochemical measurements and enable possible energy diagrams for the (Ti,V)O 2 alloys (fig. 5). Also, with these diagrams those of TiO 2 and VO 2 [4,5,18] are shown. Vanadium atoms substituted for titanium atoms create energy levels localized below the condyction band "rr* of TiO 2. This leads to a fundamental transition of smaller energy value. When [V]/[Ti] is sufficiently large, these levels result in a new and distinct energy band as in VO 2. These assumptions are correlated to the

J. Gautron et al. / Ceramic (Ti, V)O 2 and (Ti, Nb)O 2 alloys

109

Vd

--2

H20/H 2Lu,~~,

---1

pH=t4

A uJ

ph( )current >

OH/O

mO uJ

2. . . . . . .

Eg= 3eV

Eg = 2.22eV

Eg = 2.13eV

I

> 5--

Eg = 2.04 eV

w

6--- 1

x=O

x=0.4

x=045

x=0.6

x= 1

Fig. 5. Energy diagrams of the Ti~ _xVxO 2 alloys as suggested by the experiments. The vanadium levels are located below the ,rr* band (Tid). The position of photocurrent onset is also reported and agrees with the energy diagrams. The improbability of splitting H 2 0 is also indicated.

modifications of the action spectrum as x changes. For instance at x = 0.60, the action spectrum is similar in shape to that of TiO 2 which characterizes a true interband transition. In the VO 2 case, the broadening of the spectrum towards the near infrared, together with the more cathodic value of l/on indicates that the fundamental transition takes place between two d-bands. We notice that the hypothesis concerning the possibility of a d-band into the alloys with x < 1 is in good agreement with the V - V pairs whose existence has been mentioned by H 6 d i n et al. [19] in this region of the composition range. For the (Ti,Nb)O 2 alloys to the contrary Shin et al. [20] pointed out that as soon as [ T i ] / [ N b ] = 0 . 1 7 , the N b - N b pairs are no longer stable. This results in a delocalization of the 4d 1 electrons of the Nb 4+ ions which minimizes their influence

110

J. Gautron et a L / Ceramic (Tt, V)O: and (Ti, Nb)O: allqv,s

in the reflectivity spectrum which becomes qualitatively similar to that of TiO 2 [6]. These considerations are in good agreement with our photoelectrochemical measurements. Thus, the iph versus V characteristics of Ti~_ , N b , O 2 remain similar to that of T i O 2 for 0 < x < 0.80. More surprising is the fact that the m a i n transition we observe, in the whole range of composition, still corresponds to an energy difference of = 3 eV in spite of the i n t r o d u c t i o n of N b levels. F u r t h e r m o r e , we notice that the energy value of the transition in N b O 2 c a n n o t be interpreted by m e a n s of the energy diagram in fig. 1. A possible e x p l a n a t i o n may be in the difficulty to detect very small p h o t o c u r r e n t s which are possibly related to transitions between narrow energy b a n d s (small a b s o r p t i o n coefficient). Such an hypothesis could be verified by experiments at lower temperatures where the m o b i l i t y of the carriers is expected to increase. With a n energy gap of = 2 eV a n d good stability, the (Ti,V)O 2 alloys seem attrative to solar energy conversion. However, other characteristics are in conflict with such utilization. First, the location of their flat b a n d potential hinders hydrogen evolution. The second characteristic is more severe because it concerns the conversion efficiency. The very low values we o b t a i n e d are certainly due to the quality of o u r materials (density, homogeneity . . . . ). Also it appears to be the inherent tendency. So, as one tries to extend the p h o t o r e s p o n s e of TiO 2 toward the visible by alloying (or doping) then one enhances the r e c o m b i n a t i o n s and the p h o t o c u r r e n t goes down. Besides energy conversion, one notes that the stability, m e n t i o n e d above does not hold for VO 2 which decomposes strongly. This can be a t t r i b u t e d to the nature of the f u n d a m e n t a l t r a n s i t i o n associated to the p h o t o c u r r e n t in VO2: d---, d whereas in T i O 2 it is a p ~ d. The good stability of N b O 2 may be correlated also to a f u n d a m e n t a l p --, d transition. T h a t added to the d e t e r m i n a t i o n of the energy gap a n d the type of conductivity emphasizes the role of photoelectrochemistry in the knowledge of properties of new materials.

References [1] (a) J. Gautron, P. Lemasson and J.F. Marucco, Faraday Disc. Roy. Soc. 70 (1980) 81. (b) P. Lemasson, Faraday Disc. Roy. Soc. 70 (1980) 127. [2] See e.g.J. Augustinski, J. Hinden and Chs. Stalder, J. Electrochem. Soc. 124 (1977) 1063. J.F. Houlihan, D.B. Armitage, T. Hoovler and D. Bonaquist, Mater. Res. Bull. 13 (1978) 1205. H.P. Maruska and A.K. Ghosh, Solar Energy Mater. 1 (1979) 237. Y. Matsumoto, J. Kurimoto, T. Shimizu and E. Sato, J. Electrochem. Soc. 128 (1981) 1040. [3] (a) W. Rtidorff, G. Walter and J. Stadler, Z. Anorg. Allgem. Chem. 297 (1958) 1. (b) K. Sakata, J. Phys.. Soc. Japan 26 (1969) 1067. [4] J.B. Goodenough, in: Progress in Solid State Chemistry, vol. 5, ed. H. Reiss (Pergamon, Oxford. 1971) pp. 235, 351. [5] A. Zylberstein and N.F. Mort, Phys. Rev. BI1 (1975) 4383. [6] S.M. Lu, S.H. Shin, F.H. Pollak and P.M. Raccah, in: Proc. 13th Intern. Conf. Physics of Semiconductors, Rome, 1976, ed. F.G. Fumi (North-Holland, Amsterdam, 1976). [7] J.F. Marucco, B. Poumellec, J. Gautron and P. Lemasson, unpublished results.

J. Gautron et aL / Ceramic (Ti, 1I)02 and (Ti, Nb)O 2 alloys

[8] [9] [10] [11] [12] [ 13] [14] [15] [16] [17] [18] [19] [20]

111

B. Touzelin, private communication. T.E. Phillips, K. Moorjani, J.C. Murphy and T.O. Poehler, J. Electrochem. Soc. 129 (1982) 1210. W.W. G/irtner, Phys. Rev. 116 (1959) 84. P. Lemasson, A. Etcheberry and J. Gautron, Electrochim. Acta 27 (1982) 607. N. Daude, G. Gout and C. Jouanin, Phys. Rev. BI5 (1977) 3229. J.G. Mavroides, J.C. Fan and H.J. Zeiger, in: Photoeffects at Semiconductor-Electrolyte Interfaces, ed. A.J. Nozik, ACS, Symp. Series 146 (American Chemical Society, Washington, DC, 1981) p. 217. F.J. Morin, Phys. Rev. Lett. 3 (1959) 34. R.F. Janninck and D.M. Whitmore, J. Phys. Chem. Solids 27 (1966) 1183. C.N.R. Rao, M. Natarajan, G.V. Subba Rao and R. Lochman, J. Phys. Chem. Solids 32 (1971) 1147. K. Sakata, I. Nishida, M. Matsushima and T. Sakata, J. Phys. Soc. Japan 27 (1969) 506. E. Caruthers and L. Kleinman, Phys. Rev. B7 (1973) 3760. T. H6rlin, T. Niklewski and M. Nygren, J. de Phys. 37 C4 (1976) 69. S.H. Shin, T.H. Halpern and P.M. Raccah, Mater. Res. Bull. 10 (1975) 1061.