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Photocatalytic hydrogen production with Cd(S, Se) solid solution particles: Determining factors for the highly efficient photocatalyst
VoIume
CHEMICAL
109. number 1
PHOTOCATALYTIC DET~~~~G
PHYSICS
LETTERS
3 August 1984
HYDROGEN PRODUCTION WITH Cd@, Se) SOLiD SOLUTION PARTICLES:
FACTORS FOR THE HIGHLY EFFICIENT PHOTOCATALYST
Shiro KAhIBE, Masatoshi
FUJII, Tomoji KAWAI, Shichio KAWAI
TIte Itntiture of Sflmetttifc and Ittdustrial Research, Osaka Uniters&y, Mihogaoka, Ibaraki, Osaka 567. Japatt
and Fujiya NAKAHARA GOvertttttettr Ittdustrial Research IttsCtute. Osaka, lkeda. Osaka 563. Japan Received 18 February
1984: in final form 16 May 1984
By using CdSt _..&e_~ particles whose valence band positions are continuously variable, two determix@ factors for the photocatalytic production of Hz from water and an organic compound have been elucidated: (1) the valence band position of the semiconductor relative to the donor level of the organic compound. and (3) tnnsport properties of the particle.
l_ Introduction
a semiconductor photocatalyst is influenced by the valence band position_ it is interesting to study the photocatalytic reaction of this Cd(S, Se) solid solution to explore the relation between the reactivity and properties of the particles_ Here we report on hydrogen production from water and ethanol or other organic molecules with this particular powdered photocatalyst.
Numerous studies have been carried out on solar energy conversion with particulate semiconductor photocatalysts in order to achieve high conversion efficiency [ 1] _The most important problem is bow to exploit an efficient material for the pf~otocatalyst which works under visibie light. In order to get a highly efficient photocatalyst, the factors which control the photocatalytic activity must be elucidated_ For a semiconductor photocatalyst particle, the energy gap and the positions of the conduction and valence bands are the properties which should be first considered when choosing a material for the photocatalyst [ 1.,2]. These parameters, however, are considered to be fixed for each semiconductor material under certain conditions, such as the pHof the solution. Accordingly, we must use completely different materials for the photocatalyst if we want to change these parameters for the design of an efficient photocataly~. Fortunately, the valence band position of a Cd(S, Se) solid solution can be artificially changed and controlled by varying the composition ratio of sulfur to selenium f3]. Since the oxidizing power of a hole in 0 009~2614[84jS 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
2. Experimental
Preparaii5~~of Cd& Se) solid s~~lir~~~~ panicles_ Powders of Cd(S, Se) soIid solution of desired composition were prepared by mixing CdCO,, S and Se, followed by ball-milling and calcination at 550°C for 2 h. The crystal structure and the chemical composition were confirmed by X-ray diffractometry. Pl~otocata~vst preparariw 143_The Cd(S, Se)powder was mixed with Pt black (Engelhardt Co., 15 wt%) in an agate mor?ar to form powdered CD(S, Se)/Pt photocatalyst. f@drogerz production experimwt_ The photocatalyst (300 mg) was suspended in a deaerated water (20 ml)-ethanol (20 mi) or water (35 ml)-formic acid (5 ml) mixture in a 280 ml Pyrex glass bulb which can B.V.
105
longer than 320 mn. The glass bulb was irradiated from the bottom with the white light of a 500 W Xe lamp. Gaseous reaction products were analyzed by a gas chromatograph (Okura Co.), as described previously [4] _ Detmrit~atim ofthe tlandgap. The band gap of the Cd(S, SC) was determined by means of photoacoustic spectroscopy (PAS) [5]. Plloroc~~ildrrcril.ir~. and hrmimsccnce expcrirurnr. To estimate the carrier mobilities in pbotocatalysts information
about
trapping
centers
sunmiarized in table l_ In fig. 2 are shown the rates obtained versus the corresponding band gap energies
in
CdS, CdSc and Cd(S, Se). photoconductivities at room tmpcraturc and lun~inescence at 77 I( were measured.
3, Results and discussion The band gap energies of Cd(St __s, Se,) particles determined by PAS arc shown in the second co1u111n of table 1. The band gap energy decreases as the ratio of selenium to sulfur increases. In the tight-binding approximation, the conduction and the valence bands of CdS consist mainly of the 5s orbital of cadmium 2nd 3s3p orbitals of sulfur, respectively. As the ratio of Se/S increases in Cd(S, SC), the contribution of 4s4p orbitals of selenium to the valence band becomes dominant and rhe position of the valence band is continuously shifted upwards in this solid solution system, while the conduction band is fixed by the 5s orbital of cadmium (see fig. 1). Accordingly. the change of the band gap energy for these samples cor-
The band gap energies oiCdtS. SC) particles and the II, production reduciw-ag:enr 2
of Cd(S, Se) determined by PAS. These dependences of Hz production rate on the variation of the band gap energy are quite different for ethanol and formic acid, as shown in fig. 2. For ethanol as the reducing agent: (1) The hydrogen production rate gradually decreases as the band gap of Cd(S, Se) becomes narrower. (2) When the band gap energy falls short of 2.2 eV. H2 production stops. This value corresponds to the valence band edge being 1.4 V versus NHE, since the conduction band edge of CdS under these conditions is -0.8 V versus NHE [8]. On the other hand, for formic acid as reducing agent: (1) HZ production was observed for all the Cd(S,Se) photocatalysts. (2) The HZ production rate for each Cd(S, Se) sample becomes more than 15 times as large as that for ethanol. The quantum efficiency is 20% for CdS/Pt with light of 470 nni wavelength. (3) The smaller the band gap of Cd(S, Se) becomes,
rate with a platinized Cd(S, Se) photocatalyst
mix~urc
Cd(S. SC) Se content (2;) reducing a_gent ErOIi
106
1984
responds to that of the valence band position. In fact, the flat band potentials of CdS, CdSe and Cd(Se,_,, Te,) solid solution photoelectrodes exhibit similar values under the same conditions [6,7]. When an aqueous suspension of the platinized Cd(S, Se) powder was irradiated in the presence of ethanol or formic acid, hydrogen gas was evolved as streams of bubbles. Hydrogen production rates thus obtained for different compositions of Cd(S, Se) are
pass light of a wavelength
and to obtain
3 Auqst
CHEMICAL PHYSICS LETTERS
Volume 109. number 1
0
2.42
9230
3 4 a 9 12 25 59 100
2.41 2.40 2.35 2.33 2.30 1.1s 1.96 1.68
1760 1410 820 168 105 6 6 0
reducing agent 1ICOOH 135000
30800 28100 20400 19700 16700 16800 17600 IKxm
in a water-
Volume
109, number
CHEMICAL
1
PHYSICS
LEl-i-ERS
the less H7 is produced,
Cd+,$ex
3 Ausst
1984
and the rate for CdSe drastic-
ally increases again. -1 _
. ..e
. . I
CdSs
C.B.
Fig. 1. Energy scheme for tbe Cd(Sr __s, Se,-) photocatalyst particle. As the Se/S ratio increases, the top of 111~vatencc band is continuously shifted upwards. The donor levels of ethanol and formic acid are shown at the right.
l$
Pro& rote
i0
In these reaction systems, the organic compound is osidized by the hole at the valence band and HZ praduction is driven by the electron at the conduction band. When the electron energy in tile highest occupied molecular orbital (HOMO) of EtOH is higher than the edge of the valence band of Cd(S, Se). EtOH can be osidized by the hole at the valence band of Cd(S. Se) and H7 production occurs. When the HOMO IeveI of EtOH 7s lower than the valence band of Cd@, Se). HZ production cannot occur. Accordingly, the dependence of Hz production from the ethanol-water mixture in fig. 2 suggests that the electron energy of rhe EtOH molecule on the semiconductor surface locates near the valence band edge of CdS, i-e_ about 1.3 V versus NHE. As the osidation potential of EtOH measured on a Pt electrode (l/3 peak: 2.9 V versus NHE) [9.10] is much deeper than the energy of the valence band of Cd& Se), as shown in fig. 1, the upward energy shift of about I .5 eV is considered to be caused by the adsorption of EtOH on the Cd(S, Se) surface. When a molecule is adsorbed on a semiconductor
CdtS,Se)/Pt
Rattkxtalyst
3
6
4
2
i
0
i.s
2.0
2.2
cze Band 9OD enew
2.1( cas
/ eV
Fig. 2. The band gap energies of Cd& Se) particIes and the H2 production rate with a platiaized Cd(S. Se) photocatalyst in an aqueous ethanol or formic acid solution.
sur-
face, the electron energy of the molecule shifts up due to the adsorption if the electron energy of the molecule is more positive than the Fermi level of rile semiconductor [ 11 .12]_ The H, production rates with HCOOH instead of EtOH as reducing agent show a drastic increase, as shown in table 1. The oxidation potential of formic acid is I.68 V versus NHE [IO]. This value is much more negative than that of EtOH. Accordingly, the electrons of formic acid can be efficiently extracted by the hole at the valence band of Cd(S, Se) so rhat the recombination of electron and hole inside a particle is reduced_ This leads to the highly efficient HZ production rate for formic acid compared with that for &OH. This expiains the fact that HZ is produced from aqueous HCOOH for aII the Cd& Se) samples with a higher production rate than that from ethanol. Interestingly, CdSe/Pt S!IOWS much higher activity for the decomposition of HCOOH than does Cd(S, Se)/Pt, as shown in tIg_ 2. This phenomenon cannot be explained by a consideration of the relative energy positions of HCOOH to the valence band. When the redox potential of HCOOH is much more negative than the valence band position, the reducing power of the reagent to the hole would not be strongIy affected IO7
CHE~UCAL PHYSICS LETTERS
Volume 109, numixlr 1
by a slight change of the valence band position, or at least the photocatalytic activity is expected to change monotonically as the position of the valence band gradually becomes shallower. Therefore, the low activity of the Cd@, Se) solid solution in fig. 2 may be deduced IO be a property of these particular photocatalysts. In 3 semiconductor particle. the irradiation leads 10 the formation of electrons and holes, followed by charge separation. migration of charge carriers to the surface. and the reaction with solution_ Accordingly, changes of the transport properties of the particles may affect the photocatalytic activity of the particles. The photoconductivity measurement revealed the changes of the carrier mobility in the photocatalysts
is
i.e. the Hz production rate decreased monotonically. It should be noted that, with the choice of a proper composition of Cd(S, Se), the selectivity for the oxidation of C7H50H and HCOOH can be controlled, as shown in fii. 2. With CdSe and Cd(S,, Se,_,)(O <_u <0.X), HCOOH alone is oxidized, while both
Thus, we can deduce
C2H50H and HCOOH are oxidized with CdS or Cd(S,, Set __,)(0.25 <_x < l)_ The activity of C,H50H oxi-
be due to trapping centers caused by impurities in solid solution particles. In fact, the luminescence exat 77 K eshibited
an emission
peak
at 650
mn for particles of solid solution. Since this energy less than the band gap energies. the peak is presumably due to a trace
of inipuritics.
t!Iat the lower reactivity of Cd(S. Se) particles for I-K0011 in fig. 2 is due to the decrease of the carrier mobility in particles of Cd(S. Se) solid solution. Taking advantage of the fact that the valence band
Hz production
I’lIotocatal~st
rate with various
Band g!al, (eV) z_
cncr-1’
--__.___ CdS Cd(S. Se) Cd(S. SC) Cd% a) +: hydrogen
108
2.41 2.33 1.98 1.68 was detected.
kinds
1984
position of Cd(S, _-x, Se,) solid solution can be artificially controlled by changing x, the following two determining factors for Hz production have been elucidated: (1) The relative position of the donor level of a reducing agent to the valence band of a semiconductor particle. This factor is dominant for the reducing agent whose donor level is close to the valence band edge of the semiconductor, e-i. ethanol. (2) Transport properties of the particle, such as carrier mobility. This factor is dominant for the reducing agent whose donor level is much more negative than the valence band edge, i.e. HCOOH. This shows that ~by using purer photocatalysts of solid solution, we can get higher Hz production efficiency. Finally, we would like to emphasize that these two basic determining factors could be estended to other reducing agents_ as shown in table 2. Oxalic acid, whose redox potential is relatively negative, showed similar behavior to HCOOH, i.e. the H, production rate decreased for Cd(S, Se) solid solution. Methanol or Na2S203, whose redox potential is relatively positive, showed similar behavior to C,H,OH.
used in this study. The photoconductivities of paticles of Cd(S, Se) solid solution wcrc revealed to be less than one fifteenth as nlucl~ as those of CdS or CdSe. Supposing that the number of carriers created under irradiation is constant with all samples. the carrier mobility of Cd(S. Se) solid solutions is much smaller than that of CdS or Cd%. The decrease of carrier mobility makes the recombination rate of electrons and holes larger. so that the charge becomes unable to be used in the reaction. The decrease in the photoconductivity may
pcrimcnt
3 Ausst
of reducing
agents
dation is increased with an increase of the ratio of sulfur to selenium. The controlled selectivities using this particular Cd(S, Se) photocatalyst may be interesting from the viewpoint of organic synthesis.
a)
Reducing agents and redos -
potential
(V versus
---
NIW)
(COOH)2. -0.49
HCOOH, -0.199
CH30H, 0.190
&HsOH,
10900 600 600 4300
135000 19700 17600 82000
300 40 + 0
9100 270 + 0
0.192
Na2S,03, 200 + 0 0
0.4
Volume 109, number 1
CHEMICAL PHYSICS LETTERS
Acknowledgement This work is partly supported by a grant from the research project in ISIR on new materials for efficient energy conversion. We would like to express our appreciation to Dr. T. Taguchi for the luminescence esperiments and valuable discussions. Thanks are also
due to Dr. 0. Nakamura for the PAS measurements.
3 Aqust 1981
141 T. Kiawai and T. Sakara. J. Chem. Sot. Chem. Commun. (1980) 694. [S] A. Rosencwa~. Photoacoustics and photoacoustic
spectroscopy (Wiley. New York, 1980) ch. la_;_ 161 T.N. Veziroglu. Ii. Fueki and T. Ohta. eds_. Hydrogen encrzv progress, Vol. 2 (Peqamon, Oxford, f 980) p. 753. f 7 1 J.S. Connolly. cd., Phozochcmicat conversion and sronge of solar energy (Academic Press. New York. 1981) p. 419. [ 8 ] Xl. Gleria and R. 1femmin:. J. Electroanal. Chem. 65
(1975) 163. 191 1v.X Latimer. Oxidation potentials. 2nd Ed. (PrenriceReferences f I] ;\I.Ctitzel, ed.. Energy resources through photochemistry and catalysis (ilcadcmic Press, New York. 1983). [?] A.J. Bard, J. Phys. Chem. 172 (1987) 86. 131 h1.P. Lisitsa, V.N. Mali&o and S-F_ Terckhow. Soviet Phvs. Semicond. 3 (1969) 491.
Hall, Erglewood Cliffs. 1951) ch. S. 1101 A.J. Bard and H. Lund, edr, Encyciopaedis of elecrrochemistry of the elements. Vol. 8 (Dckker. New York. 1979) ch. I. [ 1 l] B. Feubacher and B. Firton, in: Topics in current physics, Vol. 4. Electron spectroscopy for surface analysis. ed. H. Ibach (Springer_ Berlin. 19771 ch. 5.~ f 12 j J. Benard. ed.. Adsorption on metal surfaces. rtn intcgated approach (Elsevier. -Amsrerdzun, 1953) ch. 6.2.
109
CHEMICAL
109. number 1
PHOTOCATALYTIC DET~~~~G
PHYSICS
LETTERS
3 August 1984
HYDROGEN PRODUCTION WITH Cd@, Se) SOLiD SOLUTION PARTICLES:
FACTORS FOR THE HIGHLY EFFICIENT PHOTOCATALYST
Shiro KAhIBE, Masatoshi
FUJII, Tomoji KAWAI, Shichio KAWAI
TIte Itntiture of Sflmetttifc and Ittdustrial Research, Osaka Uniters&y, Mihogaoka, Ibaraki, Osaka 567. Japatt
and Fujiya NAKAHARA GOvertttttettr Ittdustrial Research IttsCtute. Osaka, lkeda. Osaka 563. Japan Received 18 February
1984: in final form 16 May 1984
By using CdSt _..&e_~ particles whose valence band positions are continuously variable, two determix@ factors for the photocatalytic production of Hz from water and an organic compound have been elucidated: (1) the valence band position of the semiconductor relative to the donor level of the organic compound. and (3) tnnsport properties of the particle.
l_ Introduction
a semiconductor photocatalyst is influenced by the valence band position_ it is interesting to study the photocatalytic reaction of this Cd(S, Se) solid solution to explore the relation between the reactivity and properties of the particles_ Here we report on hydrogen production from water and ethanol or other organic molecules with this particular powdered photocatalyst.
Numerous studies have been carried out on solar energy conversion with particulate semiconductor photocatalysts in order to achieve high conversion efficiency [ 1] _The most important problem is bow to exploit an efficient material for the pf~otocatalyst which works under visibie light. In order to get a highly efficient photocatalyst, the factors which control the photocatalytic activity must be elucidated_ For a semiconductor photocatalyst particle, the energy gap and the positions of the conduction and valence bands are the properties which should be first considered when choosing a material for the photocatalyst [ 1.,2]. These parameters, however, are considered to be fixed for each semiconductor material under certain conditions, such as the pHof the solution. Accordingly, we must use completely different materials for the photocatalyst if we want to change these parameters for the design of an efficient photocataly~. Fortunately, the valence band position of a Cd(S, Se) solid solution can be artificially changed and controlled by varying the composition ratio of sulfur to selenium f3]. Since the oxidizing power of a hole in 0 009~2614[84jS 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
2. Experimental
Preparaii5~~of Cd& Se) solid s~~lir~~~~ panicles_ Powders of Cd(S, Se) soIid solution of desired composition were prepared by mixing CdCO,, S and Se, followed by ball-milling and calcination at 550°C for 2 h. The crystal structure and the chemical composition were confirmed by X-ray diffractometry. Pl~otocata~vst preparariw 143_The Cd(S, Se)powder was mixed with Pt black (Engelhardt Co., 15 wt%) in an agate mor?ar to form powdered CD(S, Se)/Pt photocatalyst. f@drogerz production experimwt_ The photocatalyst (300 mg) was suspended in a deaerated water (20 ml)-ethanol (20 mi) or water (35 ml)-formic acid (5 ml) mixture in a 280 ml Pyrex glass bulb which can B.V.
105
longer than 320 mn. The glass bulb was irradiated from the bottom with the white light of a 500 W Xe lamp. Gaseous reaction products were analyzed by a gas chromatograph (Okura Co.), as described previously [4] _ Detmrit~atim ofthe tlandgap. The band gap of the Cd(S, SC) was determined by means of photoacoustic spectroscopy (PAS) [5]. Plloroc~~ildrrcril.ir~. and hrmimsccnce expcrirurnr. To estimate the carrier mobilities in pbotocatalysts information
about
trapping
centers
sunmiarized in table l_ In fig. 2 are shown the rates obtained versus the corresponding band gap energies
in
CdS, CdSc and Cd(S, Se). photoconductivities at room tmpcraturc and lun~inescence at 77 I( were measured.
3, Results and discussion The band gap energies of Cd(St __s, Se,) particles determined by PAS arc shown in the second co1u111n of table 1. The band gap energy decreases as the ratio of selenium to sulfur increases. In the tight-binding approximation, the conduction and the valence bands of CdS consist mainly of the 5s orbital of cadmium 2nd 3s3p orbitals of sulfur, respectively. As the ratio of Se/S increases in Cd(S, SC), the contribution of 4s4p orbitals of selenium to the valence band becomes dominant and rhe position of the valence band is continuously shifted upwards in this solid solution system, while the conduction band is fixed by the 5s orbital of cadmium (see fig. 1). Accordingly. the change of the band gap energy for these samples cor-
The band gap energies oiCdtS. SC) particles and the II, production reduciw-ag:enr 2
of Cd(S, Se) determined by PAS. These dependences of Hz production rate on the variation of the band gap energy are quite different for ethanol and formic acid, as shown in fig. 2. For ethanol as the reducing agent: (1) The hydrogen production rate gradually decreases as the band gap of Cd(S, Se) becomes narrower. (2) When the band gap energy falls short of 2.2 eV. H2 production stops. This value corresponds to the valence band edge being 1.4 V versus NHE, since the conduction band edge of CdS under these conditions is -0.8 V versus NHE [8]. On the other hand, for formic acid as reducing agent: (1) HZ production was observed for all the Cd(S,Se) photocatalysts. (2) The HZ production rate for each Cd(S, Se) sample becomes more than 15 times as large as that for ethanol. The quantum efficiency is 20% for CdS/Pt with light of 470 nni wavelength. (3) The smaller the band gap of Cd(S, Se) becomes,
rate with a platinized Cd(S, Se) photocatalyst
mix~urc
Cd(S. SC) Se content (2;) reducing a_gent ErOIi
106
1984
responds to that of the valence band position. In fact, the flat band potentials of CdS, CdSe and Cd(Se,_,, Te,) solid solution photoelectrodes exhibit similar values under the same conditions [6,7]. When an aqueous suspension of the platinized Cd(S, Se) powder was irradiated in the presence of ethanol or formic acid, hydrogen gas was evolved as streams of bubbles. Hydrogen production rates thus obtained for different compositions of Cd(S, Se) are
pass light of a wavelength
and to obtain
3 Auqst
CHEMICAL PHYSICS LETTERS
Volume 109. number 1
0
2.42
9230
3 4 a 9 12 25 59 100
2.41 2.40 2.35 2.33 2.30 1.1s 1.96 1.68
1760 1410 820 168 105 6 6 0
reducing agent 1ICOOH 135000
30800 28100 20400 19700 16700 16800 17600 IKxm
in a water-
Volume
109, number
CHEMICAL
1
PHYSICS
LEl-i-ERS
the less H7 is produced,
Cd+,$ex
3 Ausst
1984
and the rate for CdSe drastic-
ally increases again. -1 _
. ..e
. . I
CdSs
C.B.
Fig. 1. Energy scheme for tbe Cd(Sr __s, Se,-) photocatalyst particle. As the Se/S ratio increases, the top of 111~vatencc band is continuously shifted upwards. The donor levels of ethanol and formic acid are shown at the right.
l$
Pro& rote
i0
In these reaction systems, the organic compound is osidized by the hole at the valence band and HZ praduction is driven by the electron at the conduction band. When the electron energy in tile highest occupied molecular orbital (HOMO) of EtOH is higher than the edge of the valence band of Cd(S, Se). EtOH can be osidized by the hole at the valence band of Cd(S. Se) and H7 production occurs. When the HOMO IeveI of EtOH 7s lower than the valence band of Cd@, Se). HZ production cannot occur. Accordingly, the dependence of Hz production from the ethanol-water mixture in fig. 2 suggests that the electron energy of rhe EtOH molecule on the semiconductor surface locates near the valence band edge of CdS, i-e_ about 1.3 V versus NHE. As the osidation potential of EtOH measured on a Pt electrode (l/3 peak: 2.9 V versus NHE) [9.10] is much deeper than the energy of the valence band of Cd& Se), as shown in fig. 1, the upward energy shift of about I .5 eV is considered to be caused by the adsorption of EtOH on the Cd(S, Se) surface. When a molecule is adsorbed on a semiconductor
CdtS,Se)/Pt
Rattkxtalyst
3
6
4
2
i
0
i.s
2.0
2.2
cze Band 9OD enew
2.1( cas
/ eV
Fig. 2. The band gap energies of Cd& Se) particIes and the H2 production rate with a platiaized Cd(S. Se) photocatalyst in an aqueous ethanol or formic acid solution.
sur-
face, the electron energy of the molecule shifts up due to the adsorption if the electron energy of the molecule is more positive than the Fermi level of rile semiconductor [ 11 .12]_ The H, production rates with HCOOH instead of EtOH as reducing agent show a drastic increase, as shown in table 1. The oxidation potential of formic acid is I.68 V versus NHE [IO]. This value is much more negative than that of EtOH. Accordingly, the electrons of formic acid can be efficiently extracted by the hole at the valence band of Cd(S, Se) so rhat the recombination of electron and hole inside a particle is reduced_ This leads to the highly efficient HZ production rate for formic acid compared with that for &OH. This expiains the fact that HZ is produced from aqueous HCOOH for aII the Cd& Se) samples with a higher production rate than that from ethanol. Interestingly, CdSe/Pt S!IOWS much higher activity for the decomposition of HCOOH than does Cd(S, Se)/Pt, as shown in tIg_ 2. This phenomenon cannot be explained by a consideration of the relative energy positions of HCOOH to the valence band. When the redox potential of HCOOH is much more negative than the valence band position, the reducing power of the reagent to the hole would not be strongIy affected IO7
CHE~UCAL PHYSICS LETTERS
Volume 109, numixlr 1
by a slight change of the valence band position, or at least the photocatalytic activity is expected to change monotonically as the position of the valence band gradually becomes shallower. Therefore, the low activity of the Cd@, Se) solid solution in fig. 2 may be deduced IO be a property of these particular photocatalysts. In 3 semiconductor particle. the irradiation leads 10 the formation of electrons and holes, followed by charge separation. migration of charge carriers to the surface. and the reaction with solution_ Accordingly, changes of the transport properties of the particles may affect the photocatalytic activity of the particles. The photoconductivity measurement revealed the changes of the carrier mobility in the photocatalysts
is
i.e. the Hz production rate decreased monotonically. It should be noted that, with the choice of a proper composition of Cd(S, Se), the selectivity for the oxidation of C7H50H and HCOOH can be controlled, as shown in fii. 2. With CdSe and Cd(S,, Se,_,)(O <_u <0.X), HCOOH alone is oxidized, while both
Thus, we can deduce
C2H50H and HCOOH are oxidized with CdS or Cd(S,, Set __,)(0.25 <_x < l)_ The activity of C,H50H oxi-
be due to trapping centers caused by impurities in solid solution particles. In fact, the luminescence exat 77 K eshibited
an emission
peak
at 650
mn for particles of solid solution. Since this energy less than the band gap energies. the peak is presumably due to a trace
of inipuritics.
t!Iat the lower reactivity of Cd(S. Se) particles for I-K0011 in fig. 2 is due to the decrease of the carrier mobility in particles of Cd(S. Se) solid solution. Taking advantage of the fact that the valence band
Hz production
I’lIotocatal~st
rate with various
Band g!al, (eV) z_
cncr-1’
--__.___ CdS Cd(S. Se) Cd(S. SC) Cd% a) +: hydrogen
108
2.41 2.33 1.98 1.68 was detected.
kinds
1984
position of Cd(S, _-x, Se,) solid solution can be artificially controlled by changing x, the following two determining factors for Hz production have been elucidated: (1) The relative position of the donor level of a reducing agent to the valence band of a semiconductor particle. This factor is dominant for the reducing agent whose donor level is close to the valence band edge of the semiconductor, e-i. ethanol. (2) Transport properties of the particle, such as carrier mobility. This factor is dominant for the reducing agent whose donor level is much more negative than the valence band edge, i.e. HCOOH. This shows that ~by using purer photocatalysts of solid solution, we can get higher Hz production efficiency. Finally, we would like to emphasize that these two basic determining factors could be estended to other reducing agents_ as shown in table 2. Oxalic acid, whose redox potential is relatively negative, showed similar behavior to HCOOH, i.e. the H, production rate decreased for Cd(S, Se) solid solution. Methanol or Na2S203, whose redox potential is relatively positive, showed similar behavior to C,H,OH.
used in this study. The photoconductivities of paticles of Cd(S, Se) solid solution wcrc revealed to be less than one fifteenth as nlucl~ as those of CdS or CdSe. Supposing that the number of carriers created under irradiation is constant with all samples. the carrier mobility of Cd(S. Se) solid solutions is much smaller than that of CdS or Cd%. The decrease of carrier mobility makes the recombination rate of electrons and holes larger. so that the charge becomes unable to be used in the reaction. The decrease in the photoconductivity may
pcrimcnt
3 Ausst
of reducing
agents
dation is increased with an increase of the ratio of sulfur to selenium. The controlled selectivities using this particular Cd(S, Se) photocatalyst may be interesting from the viewpoint of organic synthesis.
a)
Reducing agents and redos -
potential
(V versus
---
NIW)
(COOH)2. -0.49
HCOOH, -0.199
CH30H, 0.190
&HsOH,
10900 600 600 4300
135000 19700 17600 82000
300 40 + 0
9100 270 + 0
0.192
Na2S,03, 200 + 0 0
0.4
Volume 109, number 1
CHEMICAL PHYSICS LETTERS
Acknowledgement This work is partly supported by a grant from the research project in ISIR on new materials for efficient energy conversion. We would like to express our appreciation to Dr. T. Taguchi for the luminescence esperiments and valuable discussions. Thanks are also
due to Dr. 0. Nakamura for the PAS measurements.
3 Aqust 1981
141 T. Kiawai and T. Sakara. J. Chem. Sot. Chem. Commun. (1980) 694. [S] A. Rosencwa~. Photoacoustics and photoacoustic
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