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Correlation of photosensitive electrode properties with electronegativity
Volume 47, number 2
CHEMICAL PHYSICS LETTERS
CDRRELATION OF P~DTDSENS~~
15 April 1977
ELECTRODE PROPERTIES WITH ELECTRDNEGATfVffY*
M.A. BUTLER and D.S. GINLEY Sandia Laboratories, Aibuquerque,
New Mexico 87115. USA
Received 11 October 1976 Revised manuscript received 8 December 1976 The ability of semiconducting eiectrodes to photoelectrolyze water without any extemti bias is an important criterion for satisfactory conversion of solar energy to chemical energy. Here we correlate this ability with the electronegativities of the constituent atoms.
The photoe~ectroIysis of water using semiconducti~g electrodes has recently attracted considerable attention. The possibility of using sunlight to produce Hz is very attractive and an extensive search is underway to find the optimum material [I,21 . Thus far, the search has been a rather unstructured affair. In this paper, we discuss some possible guidelines for electrode selection and consider the importance of the electronegativity of the constituent atoms in determining their currentvoltage characteristics. One important criterion for the ideal semiconducting electrode is a suitable band structure such that the eleetrochemica! cell operates at zero bias (short circuit conditions). While most semiconducting electrodes appear to work if a bias is applied, the presence of a power supply significantly reduces the overall efficiency of the system. The energy level diagram for an electrochemicaI cell with an n-type semiconducting electrode is represented schematically in fig. 1, If there are no surface states present, then the amount of band bending in the semiconductor, which determines the depletion layer width and the magnitude of the electric field available to separate the electron-hole pairs, is set by the semiconductor electron affinity, EA. Clearly, for optimum efficiency at short circuit conditions, one wishes as small an EA as possible [2]. There has been considerable recent interest in predicting the Fermi energies and photoelectric thresholds * Prepared for the U.S. Energy Reseamh and Development Admkdstration under Contract AT(29-l)-789,
Fig. 1. Energy Leveldiagram for the photoefectroIy.sis of water under short circuit conditions The relationship between electron affinity, EA, and fiatband potential, Eg,. is sfiown as welI as the reaction potentials for the oxygen and hydrogen evolution half-cell reactions. Eo is the energy diierence between the vacuum level and the saturated caiomel reference electrode-
for solids and, therefore, the electron affinity-from the electronegativities of the constituent atoms [3,4] _The approaches are generally based on M~~iken’s defiition of the eIectrone~ti~ty of a neutral atom as the arithmetic mean of the first ionization energy (Q and the electron affinity (EA) x = $(1t + EA). It is possible to extend this defmition to other valence states by taking the arithmetic mean of the energy required to add an electron and to remove an eIectron 319
15 April 1937
CHEMICAL PHYSICS LETI’ERS
Volume 47, number 2
from that valence state. Thus for a monovalent ion we have x(M* j = i (l, + lz) ,
(2)
where II and I2 are the first and second ionization potentials of the M atom. In some sense, the electronegativity is a measure of the electrochemical potential of-an electron in an atom. Thus for a compound, one would expect an equilization of the electrochemical potentials of the constituent atoms by charge transfer, Sanderson has previously postulated that just such a process occurs [S] . Sanderson [S] and more recently Nethercot 131 expect the resulting electrochemical potential for the compound (Fermi energy) to be the geometric mean of the constituents. If we accept this hypothesis and the definition of MuIliken electronegativity (atomic electrochemical potential), then we can, in principle, calculate the Fermi energies of solids from the atomic ionization energies and electron affinities. This has been done with surprising success for several classes of compounds [3,4J We have tried to utilize these concepts in an attempt to model the photoelectrolytic behavior of a series of n-type titanates of the folm MTiO,, where M is a divalent cation as listed in table 1. III the process of examining these materiaIs as anodes for the photoelectrolysis of water, we have measured their bandgaps (IQ by examining the wavelength dependence of the photocurrent and their flatband potentials (Efi) (a meas;lre of their electron affinity) by examining the potential dependence of the photocurrent [6] (see fig. 1). The flatband values may differ somewhat from published vaiues determined by capacitance measurements [2,7]. However, these differences do not affect the validity of the concepts presented in this paper. Table 1 Properties of various titanatesa)
SrTiO, and BaTi03 were reduced single crysrals. PoIycrystaUine samples of MnTiO, and FeTiO, were synthesized from a melt of the appropriate mixtures of Mn02 and Ti02 and Fe,Fe203 and Ti02, respectively [S] . Electrochemical experiments were performed with a three electrode system utilizing a saturated cafomel (SCE) reference electrode and a Pt counter electrode. A PAR model 173 potentiostat was used to obtain the data. The light source was a 150 W xenon lamp with a monochromator yielding intensities of = 1 mW/cm2. We may relate the electron affinity and flatband potential as follows: EA=EO’Efb--ilE,
(3)
where E. is the energy of a saturated calomel electrode (SCE) relative to the vacuum level (4.75 eV) [9] and AE is the distance from the Fermi level of the doped semiconductor to the bottom of the conduction band. Seebeck effect measurements [lo] indicate that AEis =0.15 eV in all the samples studied. Therefore, AE will result in a small uniform shift of Ef for all compounds. The Fermi energy of the undoped semiconductor, the quantity predicted by the electronegativities, is just: Ef= EA+iEg=EO
f$Eg +-Em -LIE.
(4)
For the metal titanates the Sanderson hypothesis gives &(MTiO,)
= [x(M)x(Ti)x3(0)]
‘I5 _
(5)
Unfortunately, the electron affinities for many atoms are unknown, or at best, poor theoretical estimates. Thus, we cannot accurateIy calculate Ef for these compounds. Since the same information can be derived from the electronegativities of the appropriate neutral collection of ions [5], the same results should be obtained from Ef(MTi03) = [x(M+)x(Ti2’)x3
(O-)] Us.
(6)
By combining eqs. (2), (4) and (6) we have Electrode
Bandgap kv)
f&(versus cv)
BaTiQa SrTiO3 MnTiOs FeT103
3.18(5) 3.12(5) 3.10(S) 2.85110)
-1.22(10) -1.19(10) -O-80( 10) -0.59(3)
a) The numbers in parentheses b) In 1 M NaOH.
320
SCE)b)
.
Qt f 12)“s (eV”5) 1.722 1.755 1.874 1.888
are errors in the last digit.
$Eg + Et-,, = AE - E. + [;(I1 +12)] ‘I5
(7)
[&T’i2’) x3(0-)] “‘.
In fig. 2 we show a plot of ($Eg + Efb) measured in 1 M NaOH as a function of (II + 12)li5 for the metal ion. The solid line in fig. 2 is a least squares fit to the data and has a slope = 29(4) and an intercept = -4.7(7)
Volume
47, number 2
CHEMICAL
PHYSICS
LETTERS
15 ApriJ 1977
tively, one expects only small changes in bandgap for changes in the divalent metal ion (see table I). In these circumstances, the ability of an electrode to operate at zero bias is determined primarily by the electronegativity of the M2+ ion. From consideration of the ionization potentials of many other possrble divaient metal ions we conclude that no significant improvement over SrTiO, or BaTiO, is possible within the titanate formula. These arguments have also been successfully applied to several compounds of the form, MFe03 to prepict optimum ionic configurations for this particular group of compounds [12] _l&is shodci provide a useful guide in the search for better photosensitive semiconducting electrodes for photoelectrolysis of water. Fig. 2. Variration of the Fermi level relative to the reference electrode (5Eg + E& in 1 M NaOH as a function of the fist and second ionization potentials of M in MTiOa. See tekt for explanation of (II + fz) 1’5 behavior. The solid tine is a least squares fit of a straight line to the data.
where the numbers in parentheses are one standard error in the last digit. From eq. (7) it can be seen that these quantities are just the slope = [ix (Ti2+)x3(O-)] Us and the intercept = AE - E,. Calculation of the slope is complicated by the electron *affinity of O- being negative, and thus one cannot obtain the geometric mean for this collection of ions. A rough estimate of this number can be obtained by equating eqs. (5) and (6) and using theoretical estimates for the atomic electron affinities of the neutral atoms [I 1] _This results in a predicted slope of 3.0, which is in good agreement with the measured value. The intercept is just the difference in energy between the vacuum level and the SCE reference electrode (see fig. 1). However, the nernstian flatband pH dependence impIies direct electrochemical participation of OH-. This introduces an unknown shift in the apparent EA of the semiconductor. For this reason, no meaningful estimate of the intercept may be made. Thus we see that the Sanderson hypothesis [S] combined with Muhiken’s definition of eIectronegativity is capable of predicting trends in Fermi energies of solids. Since, in the titanates the valence and conduction bands are composed of 0 and Ti orbitah, respec-
We thank P.J. Feibelman for many helpful discussions, Rod K. Quinn and R.D. Nasby for providing the SrTiO, and BaTiO 3 electrodes, and the latter for the Seebeck effect measurements.
References A. Fujishima and K. Honda, Nature 238 (1972) 37; hf.A. Butter, R-D. Nasby and R.K. Quinn, Solid State Commun. 19 (1976) 101 I; MS. Wrighton, D.L. Morse., A.B. Ellis, D-S. Gidey and H.B. Abrahamson, J. Am. Chem. Sot. 98 (1976) 44. [2 j J-G. Mavroides, D-1. Tchernev, J.A. Kafafas and D-F. Kolesar, Mat. Res. BulL 10 (1975) 1023. [3] A-H. Nethercot Jr., Phys Rev. Letters 33 (1974) tO88[4] R-T. Poole. D.R. Williims, J-D. Riley, J.G. Jenkins, I. Liesegang and R.C.G. Lecky, Chem. Phys. Letters 36 (1975) 401. [S] R.T. Sanderson, Chemical periodicity (Reinhold, New York, 1960). (61 ALA. Butler, in preparation. [7 ] R.D. Nasby and R.K. Quinn, Mat. Rer Bull. 1 Z (1976) 985. [8 1 R.J. Baughman and D.S. Ginlcy, Mat. Res Buli. L I (19761, to be published. (91 F. Lohman, 2. Naturforsch. 22a (1967) 843_ [lo] N.F. hfott and EA. Davis, Electronic processes in noncrystalline materials (Clarendon Press, Oxford, 1971) p. 47. [ 111 O.P. Charkin and M.E. Dyatkina, Zh. Strukt. Khim. 6 (1965) 422. [ 121 M.A. Butler and D-S. Ginley, in preparation.
[l]
321
CHEMICAL PHYSICS LETTERS
CDRRELATION OF P~DTDSENS~~
15 April 1977
ELECTRODE PROPERTIES WITH ELECTRDNEGATfVffY*
M.A. BUTLER and D.S. GINLEY Sandia Laboratories, Aibuquerque,
New Mexico 87115. USA
Received 11 October 1976 Revised manuscript received 8 December 1976 The ability of semiconducting eiectrodes to photoelectrolyze water without any extemti bias is an important criterion for satisfactory conversion of solar energy to chemical energy. Here we correlate this ability with the electronegativities of the constituent atoms.
The photoe~ectroIysis of water using semiconducti~g electrodes has recently attracted considerable attention. The possibility of using sunlight to produce Hz is very attractive and an extensive search is underway to find the optimum material [I,21 . Thus far, the search has been a rather unstructured affair. In this paper, we discuss some possible guidelines for electrode selection and consider the importance of the electronegativity of the constituent atoms in determining their currentvoltage characteristics. One important criterion for the ideal semiconducting electrode is a suitable band structure such that the eleetrochemica! cell operates at zero bias (short circuit conditions). While most semiconducting electrodes appear to work if a bias is applied, the presence of a power supply significantly reduces the overall efficiency of the system. The energy level diagram for an electrochemicaI cell with an n-type semiconducting electrode is represented schematically in fig. 1, If there are no surface states present, then the amount of band bending in the semiconductor, which determines the depletion layer width and the magnitude of the electric field available to separate the electron-hole pairs, is set by the semiconductor electron affinity, EA. Clearly, for optimum efficiency at short circuit conditions, one wishes as small an EA as possible [2]. There has been considerable recent interest in predicting the Fermi energies and photoelectric thresholds * Prepared for the U.S. Energy Reseamh and Development Admkdstration under Contract AT(29-l)-789,
Fig. 1. Energy Leveldiagram for the photoefectroIy.sis of water under short circuit conditions The relationship between electron affinity, EA, and fiatband potential, Eg,. is sfiown as welI as the reaction potentials for the oxygen and hydrogen evolution half-cell reactions. Eo is the energy diierence between the vacuum level and the saturated caiomel reference electrode-
for solids and, therefore, the electron affinity-from the electronegativities of the constituent atoms [3,4] _The approaches are generally based on M~~iken’s defiition of the eIectrone~ti~ty of a neutral atom as the arithmetic mean of the first ionization energy (Q and the electron affinity (EA) x = $(1t + EA). It is possible to extend this defmition to other valence states by taking the arithmetic mean of the energy required to add an electron and to remove an eIectron 319
15 April 1937
CHEMICAL PHYSICS LETI’ERS
Volume 47, number 2
from that valence state. Thus for a monovalent ion we have x(M* j = i (l, + lz) ,
(2)
where II and I2 are the first and second ionization potentials of the M atom. In some sense, the electronegativity is a measure of the electrochemical potential of-an electron in an atom. Thus for a compound, one would expect an equilization of the electrochemical potentials of the constituent atoms by charge transfer, Sanderson has previously postulated that just such a process occurs [S] . Sanderson [S] and more recently Nethercot 131 expect the resulting electrochemical potential for the compound (Fermi energy) to be the geometric mean of the constituents. If we accept this hypothesis and the definition of MuIliken electronegativity (atomic electrochemical potential), then we can, in principle, calculate the Fermi energies of solids from the atomic ionization energies and electron affinities. This has been done with surprising success for several classes of compounds [3,4J We have tried to utilize these concepts in an attempt to model the photoelectrolytic behavior of a series of n-type titanates of the folm MTiO,, where M is a divalent cation as listed in table 1. III the process of examining these materiaIs as anodes for the photoelectrolysis of water, we have measured their bandgaps (IQ by examining the wavelength dependence of the photocurrent and their flatband potentials (Efi) (a meas;lre of their electron affinity) by examining the potential dependence of the photocurrent [6] (see fig. 1). The flatband values may differ somewhat from published vaiues determined by capacitance measurements [2,7]. However, these differences do not affect the validity of the concepts presented in this paper. Table 1 Properties of various titanatesa)
SrTiO, and BaTi03 were reduced single crysrals. PoIycrystaUine samples of MnTiO, and FeTiO, were synthesized from a melt of the appropriate mixtures of Mn02 and Ti02 and Fe,Fe203 and Ti02, respectively [S] . Electrochemical experiments were performed with a three electrode system utilizing a saturated cafomel (SCE) reference electrode and a Pt counter electrode. A PAR model 173 potentiostat was used to obtain the data. The light source was a 150 W xenon lamp with a monochromator yielding intensities of = 1 mW/cm2. We may relate the electron affinity and flatband potential as follows: EA=EO’Efb--ilE,
(3)
where E. is the energy of a saturated calomel electrode (SCE) relative to the vacuum level (4.75 eV) [9] and AE is the distance from the Fermi level of the doped semiconductor to the bottom of the conduction band. Seebeck effect measurements [lo] indicate that AEis =0.15 eV in all the samples studied. Therefore, AE will result in a small uniform shift of Ef for all compounds. The Fermi energy of the undoped semiconductor, the quantity predicted by the electronegativities, is just: Ef= EA+iEg=EO
f$Eg +-Em -LIE.
(4)
For the metal titanates the Sanderson hypothesis gives &(MTiO,)
= [x(M)x(Ti)x3(0)]
‘I5 _
(5)
Unfortunately, the electron affinities for many atoms are unknown, or at best, poor theoretical estimates. Thus, we cannot accurateIy calculate Ef for these compounds. Since the same information can be derived from the electronegativities of the appropriate neutral collection of ions [5], the same results should be obtained from Ef(MTi03) = [x(M+)x(Ti2’)x3
(O-)] Us.
(6)
By combining eqs. (2), (4) and (6) we have Electrode
Bandgap kv)
f&(versus cv)
BaTiQa SrTiO3 MnTiOs FeT103
3.18(5) 3.12(5) 3.10(S) 2.85110)
-1.22(10) -1.19(10) -O-80( 10) -0.59(3)
a) The numbers in parentheses b) In 1 M NaOH.
320
SCE)b)
.
Qt f 12)“s (eV”5) 1.722 1.755 1.874 1.888
are errors in the last digit.
$Eg + Et-,, = AE - E. + [;(I1 +12)] ‘I5
(7)
[&T’i2’) x3(0-)] “‘.
In fig. 2 we show a plot of ($Eg + Efb) measured in 1 M NaOH as a function of (II + 12)li5 for the metal ion. The solid line in fig. 2 is a least squares fit to the data and has a slope = 29(4) and an intercept = -4.7(7)
Volume
47, number 2
CHEMICAL
PHYSICS
LETTERS
15 ApriJ 1977
tively, one expects only small changes in bandgap for changes in the divalent metal ion (see table I). In these circumstances, the ability of an electrode to operate at zero bias is determined primarily by the electronegativity of the M2+ ion. From consideration of the ionization potentials of many other possrble divaient metal ions we conclude that no significant improvement over SrTiO, or BaTiO, is possible within the titanate formula. These arguments have also been successfully applied to several compounds of the form, MFe03 to prepict optimum ionic configurations for this particular group of compounds [12] _l&is shodci provide a useful guide in the search for better photosensitive semiconducting electrodes for photoelectrolysis of water. Fig. 2. Variration of the Fermi level relative to the reference electrode (5Eg + E& in 1 M NaOH as a function of the fist and second ionization potentials of M in MTiOa. See tekt for explanation of (II + fz) 1’5 behavior. The solid tine is a least squares fit of a straight line to the data.
where the numbers in parentheses are one standard error in the last digit. From eq. (7) it can be seen that these quantities are just the slope = [ix (Ti2+)x3(O-)] Us and the intercept = AE - E,. Calculation of the slope is complicated by the electron *affinity of O- being negative, and thus one cannot obtain the geometric mean for this collection of ions. A rough estimate of this number can be obtained by equating eqs. (5) and (6) and using theoretical estimates for the atomic electron affinities of the neutral atoms [I 1] _This results in a predicted slope of 3.0, which is in good agreement with the measured value. The intercept is just the difference in energy between the vacuum level and the SCE reference electrode (see fig. 1). However, the nernstian flatband pH dependence impIies direct electrochemical participation of OH-. This introduces an unknown shift in the apparent EA of the semiconductor. For this reason, no meaningful estimate of the intercept may be made. Thus we see that the Sanderson hypothesis [S] combined with Muhiken’s definition of eIectronegativity is capable of predicting trends in Fermi energies of solids. Since, in the titanates the valence and conduction bands are composed of 0 and Ti orbitah, respec-
We thank P.J. Feibelman for many helpful discussions, Rod K. Quinn and R.D. Nasby for providing the SrTiO, and BaTiO 3 electrodes, and the latter for the Seebeck effect measurements.
References A. Fujishima and K. Honda, Nature 238 (1972) 37; hf.A. Butter, R-D. Nasby and R.K. Quinn, Solid State Commun. 19 (1976) 101 I; MS. Wrighton, D.L. Morse., A.B. Ellis, D-S. Gidey and H.B. Abrahamson, J. Am. Chem. Sot. 98 (1976) 44. [2 j J-G. Mavroides, D-1. Tchernev, J.A. Kafafas and D-F. Kolesar, Mat. Res. BulL 10 (1975) 1023. [3] A-H. Nethercot Jr., Phys Rev. Letters 33 (1974) tO88[4] R-T. Poole. D.R. Williims, J-D. Riley, J.G. Jenkins, I. Liesegang and R.C.G. Lecky, Chem. Phys. Letters 36 (1975) 401. [S] R.T. Sanderson, Chemical periodicity (Reinhold, New York, 1960). (61 ALA. Butler, in preparation. [7 ] R.D. Nasby and R.K. Quinn, Mat. Rer Bull. 1 Z (1976) 985. [8 1 R.J. Baughman and D.S. Ginlcy, Mat. Res Buli. L I (19761, to be published. (91 F. Lohman, 2. Naturforsch. 22a (1967) 843_ [lo] N.F. hfott and EA. Davis, Electronic processes in noncrystalline materials (Clarendon Press, Oxford, 1971) p. 47. [ 111 O.P. Charkin and M.E. Dyatkina, Zh. Strukt. Khim. 6 (1965) 422. [ 121 M.A. Butler and D-S. Ginley, in preparation.
[l]
321