energy position Ec Ev of semiconductors

American Mineralogist, Volume 85, pages 543–556, 2000

0003-004X/00/0304–543$05.00

543

I NTRODUCTION

The importance of synthetic semiconductors to chemical and industrial processes has spurred a large research effort to understand the fundamentals of photochemical processes and to develop new photocatalysts. For example, synthetic photocatalysts can promote processes such as photodecompo-sition of organic and inorganic contaminants (Borgarello et al.1988; Brinkley and Engel 1998; Fox 1988; Pelizzetti et al. 1988;Serpone et al. 1988a, 1988b), photosynthesis of organic com-pounds from carbon dioxide and other inorganic substrates (Anpo et al. 1997; Inoue et al. 1979; Kanemoto et al. 1996),photodecomposition of water to hydrogen and oxygen (Lauermann et al. 1987; Reber and Meier 1984), and photore-duction of dinitrogen to ammonia (Augugliaro and Palmisano 1988; Bickley et al. 1988; Schrauzer and Guth 1977; Soria et al. 1991).

In contrast, there has been relatively little research on the photocatalytic properties of mineral semiconductors. There is,however, a growing recognition of the role semiconducting minerals may play as catalysts of redox reactions in natural environments and engineered systems designed to degrade haz-ardous chemicals (Schoonen et al. 1998; Selli et al. 1996;Stumm and Morgan 1995; Sulzberger 1990). Hence, the ques-

The absolute energy positions of conduction and valence bands of

selected semiconducting minerals

Y ONG X U AND M ARTIN A.A. S CHOONEN *

Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794-2100, U.S.A.

A BSTRACT

The absolute energy positions of conduction and valence band edges were compiled for about 50each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a cata-lyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge ener-gies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H 2O oxidation potential (relative to absolute vacuum scale),energies for conduction band edges are close to, or lower than, the H 2O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sul-fides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H 2O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.

tion arises of whether semiconducting minerals could promote these same processes. If so, these minerals could play an im-portant role in the fate of contaminants and the chemical com-positions of atmosphere and hydrosphere of the early earth. Although all the processes mentioned above are photo-chemical processes, there is also evidence for non-photolytic catalysis of redox reactions by semiconductors (Xu 1997; Xu and Schoonen 1995; Xu et al. 1996, 1999). Semiconductors can act as a conduit for electrons between the aqueous reac-tants. Because no illumination is needed for non-photo cata-lytic processes, this mechanism may be important beneath the photic zone in aquatic systems.

In photochemical reactions, as well as the non-photochemi-cal mechanism outlined in our earlier work, the crucial step is the transfer of electrons between the semiconductor and sorbed reactants. As pointed out by Morrison (1990), electrons can only be transferred between those energetic states in the semi-conductor and the electrolyte that are at approximately the same energy level. The energy level of energetic states of sorbates undergoing an electron transfer can be approximated by the standard redox potential (E 0), whereas relevant energy levels for a semiconductor are the top of the valence band (E V ) and the bottom of the conduction band (E C ). The relative energet-ics of E V and E C vs. E 0 is the fundamental property of an elec-trolyte/semiconductor system that dictates whether an electron transfer between the semiconductor and sorbate is feasible.Although band gap (E g ) is well known for most semicon-ductors, unfortunately, E V and E C have not been determined ac-

*E-mail: mschoonen@https://www.wendangku.net/doc/4e5211179.html,

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES

544curately for most semiconducting minerals. Furthermore, E V and E C values are often presented in ways that prevent a straight-forward comparison to the redox potentials of aqueous elec-trolytes. For example, in both the materials science and applied physics literature, it is customary to express the energy posi-tion of band edges with respect to the energy level midway in the band gap of the material (i.e., the Fermi level of the mate-rial), rather than on the absolute vacuum scale (AVS). In con-trast, geochemical and electrochemical literature typically reports standard redox potentials for aqueous redox couples with respect to the normal hydrogen electrode (NHE). To com-pare the energy levels of sorbates to the band edges of a semi-conductor, however, it is necessary to have all energy levels of interest (i.e., E 0, E C , E V ) expressed on a common energy scale,such as the absolute vacuum scale or the normal hydrogen elec-trode scale.

The objective here is to (1) provide a compilation of the absolute energy positions of valence and conduction band edges of semiconducting metal oxide and metal sulfide minerals and (2) address the relationship between these energy positions and the catalytic activities of these minerals in various heteroge-neous electron transfer processes. For those minerals for which band edge energies have not been determined experimentally,the band edge energies were calculated using an empirical re-lationship based on the electronegativity of the elements (But-ler and Ginley 1978). More extensive reviews on the interactions between semiconductor and electrolyte and photochemistry involving semiconductors can be found elsewhere (Balzani and Scandola 1989; Bockris and Khan 1993; Gr?tzel 1988; Kish 1989; Lewis and Rosenbluth 1989; Mills and Le Hunte 1997;Nozik and Memming 1996; Morrison 1990; Nozik 1978; Smith

and Nozik 1997; Stumm and Morgan 1995; Stumm and Sulzberger 1992; Waite 1990).

T HEORETICAL BACKGROUND

Energetics of semiconductor/electrolyte interfaces For the purpose of this study we briefly review the elec-tronic band structure of semiconductors and the energetics of the semiconductor/electrolyte interface. Comprehensive treat-ments of this topic are by Bockris and Khan (1993), Gr?tzel (1989), Morrison (1990), and Nozik (1978).

The electronic structure of semiconductors is characterized by the presence of a bandgap (E g ), which is essentially an en-ergy interval with very few electronic states (i.e., low density of states) between the valence band and the conduction band,which each have a high density of states (Borg and Dienes 1992). In the context of electron transfer between semiconduc-tors and aqueous redox species, it is crucial to identify the high-est occupied and the lowest unoccupied electronic levels in the semiconductor because those are the energy levels involved in the transfer. In most semiconductors, all electronic levels in the valence band are occupied whereas the levels in the con-duction band are empty. Hence, the highest occupied electronic level coincides with the top of the valence band. The energy of valence band edge, E V , is a measure of the ionization potential,I , of the bulk material. The lowest unoccupied electronic level in most semiconductors coincides with the bottom of the con-duction band. Where the band edge energy, E C , is a measure of the electron affinity, A , of the compound. The Fermi level or energy, E F , represents the chemical potential of electrons in a semiconductor. In essence, the Fermi level is the absolute elec-tronegativity, –χ, of a pristine semiconductor, a value which

F IGURE 1. (a ) Position of the conduction band edge (E C ), the valence band edge (E V ) and the intrinsic Fermi level (E F ) of a semiconductor with respect to vacuum as the zero energy reference. A is electron affinity; χ is electronegativity; I is ionization energy; E g is band gap. (b )Position of energy levels at the interface of an n-type semiconductor and an aqueous electrolyte on the absolute vacuum energy scale (AVS) and with respect to normal hydrogen electrode (NHE). The E CS and E VS represent the conduction band edge and valence band edge at the interface;U ft is the flatband potential; V H is the potential drop of the Helmholtz layer; V B is the band bending; E F is the Fermi level of the system at equilibrium; ΔE F is the difference between the Fermi level and the conduction band edge; E 0F,redox , is the standard Fermi level of the redox couple; E unoc and E oc , are energies of unoccupied states and occupied states of the redox couple; and λ

is the reorganization energy.

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES 545

corresponds to the energy halfway between the conduction and valence band edges (Fig. 1a). The relationship between band edge energies and electronegativity can be expressed as:E C = –A = –χ + 0.5 E g (1a)

and

E V = –I = –χ –0.5 E g

(1b)Incorporation of impurities in the structure of a semiconductor leads to the presence of electron acceptor state levels and/or donor state levels within the bandgap. The presence of donor or acceptor state levels changes the position of E F so that E F lies just above E V for p -type semiconductors (presence of ac-ceptor states) and E F lies just below E C for n -type semiconduc-tors (presence of donor states) (Morrison 1990).

The next step is to define the energy levels of sorbates. Aque-ous redox species exchanging electrons with a semiconductor mineral can either accept (A + e – → A –) or donate (D → D + + e –)electrons. Upon electron transfer from or to an aqueous species,the electronic structure of the species changes. Upon the accep-tance of an electron, a previously unoccupied electronic level becomes occupied, whereas upon electron donation an elec-tron is removed from an occupied level. For an electron accep-tor (A/A –), it is the energy level of the lowest unoccupied level,E unoc , that is of importance, whereas for an electron donor (D/D +)the highest occupied energy level, E oc , is of importance. Because of the polar nature of water molecules, H 2O dipoles in the solva-tion shell of a redox species will re-orientate when there is a change in the charge of the redox species. The re-orientation of the solvation shell will result in an addition energy gain or loss when an electron is transferred from or to an aqueous redox spe-cies. The free energy change associated with this re-orientation process is known as reorganization energy (λ). V alues for λ range from a few tenths of an eV up to 2 eV . Furthermore, thermal fluctuation of the solvation structure cause a corresponding ther-mal distribution of the energy levels of both E oc and E unoc , see Figure 1b (Bockris and Khan 1993; Gr?tzel 1989; Morrison 1990). Whereas these energy distributions are difficult to quan-tify, it is helpful to make use of the notion that the redox poten-tial, E redox , of a redox couple undergoing a one-electron transition (e.g., A/A – or D/D +) lies midway between the maxima in E oc and E unoc for these species. The Fermi level of electrons of a redox couple (E F,redox ) is equivalent to the redox potential of aqueous redox couples (E redox ) on the absolute energy scale; hence, this can be expressed as:

E E E T a a F,A/A A/A A/A o

A A

R ????==+ln(

)

(2a)E E E T a a F D/D D/D D/D o D D

R +,ln(

)

+++==+(2b)

where E 0 is the standard redox potential of the aqueous redox couple with respect to the Normal Hydrogen Electrode (NHE).The Fermi level of NHE at 25 °C is –4.5 eV with respect to the vacuum level (Bockris and Khan 1993).

The distribution of energy states in a redox couple (Fig. 1b)becomes more complicated if the redox couple undergoes a multi-electron tranfer, because each one-electron step has a

population of two energy states (E oc and E unoc ) associated with

it. Instead of standard redox potential of the overall reaction,the standard potential of each one-electron steps should be con-sidered for a multi-electron transfer reaction. So for a two-elec-tron transition, such as CO 2 + 2e – + 2H + = HCOOH, the standard potentials of the CO 2/CO 2·– and CO 2·–/HCOOH redox couples should be used (CO 2·– represents a CO 2 radical group with a charge of –1). The standard redox potentials for the one-elec-tron steps are mostly unavailable because the intermediate prod-ucts are often unstable. Using E 0 for the overall reaction invariably leads to a misleading comparison of energy levels between semiconductor and aqueous species. This arises from the fact that E 0 for the overall reaction does not lie midway between the maximum of E oc and E unoc populations associated with each of the one-electron transitions. For example, E 0 for

CO 2/CO 2

·–

is estimated to lie at –2 V (NHE), whereas E 0 for CO 2/HCOOH equals –0.61 V(NHE) (Tributsch 1989).

When a semiconductor is placed in a solution containing redox species, electrons will be transferred across the semi-conductor/electrolyte interface until the chemical potentials,i.e., Fermi levels, of electrons in the solid and the solution are equalized. The interfacial electron transfer generates a space charge layer in the semiconductor, and conduction and valence band edges are bent such that a potential barrier is established against further electron transfer across the interface. As a re-sult, the energies of conduction and valence band edges at the semiconductor/electrolyte interface, (E CS and E VS , respectively),deviate from their bulk values (E C and E V ). The difference be-tween E CS and E C or E V and E VS is known as band bending, V B (Fig. 1b). The thickness of the space charge layer typically ranges from 100 ? to several microns depending on the con-ductivity of the semiconductor and the amount of band bend-ing. On the solution side of the interface, the Helmholtz double layer will develop due to the sorption of counter-ions onto the charged surface of the semiconductor. The thickness of the Helmholtz layer is typically on the order of 1 ? (Morrison 1990). The Helmholtz layer results in an additional potential drop inside the semiconductor space charge layer so that the band bending adjusts to make the net rate of electron transfer across the interface equal to zero at equilibrium. Hence, the band edge positions of the semiconductor at the interface can-not be determined unless the additional potential drop associ-ated with the Helmholtz layer is quantified (Morrison 1990).To link the energy levels of the semiconductor and the elec-trolyte, an experimentally measurable quantity, flatband po-tential (U ft ), is essential. U ft is the electrode potential measured with respect to a reference electrode (e.g., normal hydrogen electrode, NHE) in an electrolyte/semiconductor system when the potential drop across the space charge layer becomes zero.U ft can be expressed as (Nozik 1978):U ft (NHE) = A + ΔE F + V H + E 0

( 3)

where ΔE F is the difference between the Fermi level and ma-jority carrier band edge (E C for a n-type semiconductor, and E V for a p-type semiconductor), V H is the potential drop across the Helmholtz layer, and E 0 is the scale factor relating the refer-ence electrode redox level to the A VS (E 0 = –4.5 V for NHE,Bockris and Khan 1993). Because U ft is determined not only

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES 546

by intrinsic properties of the semiconductor (A andΔE F), but also by the electrolyte (V H), U ft is a property of the interface.

Note that V H is fixed and independent of both the externally applied voltage across the semiconductor-electrolyte interface and the changes in redox condition of the system, provided the composition changes associated with electron transfer do not affect the equilibrium distribution of ions adsorbed onto the semiconductor. The independence of V H on the interfacial charge transfer is caused by the high charge density and small width of the Helmholtz layer relative to the semiconductor space charge layer (Morrison 1990). As a result, the potential drop across the interface caused by the electron transfer occurs pre-dominantly within the semiconductor space charge layer, whereas the V H remains essentially constant. Consequently, at a given electrolyte composition and semiconductor, U ft is a characteristic parameter independent of the electron transfer process. On the other hand, the potential drop within the Helmholtz layer depends on the adsorption/desorption equi-librium of electrolyte ions at the semiconductor surface. When the net adsorbed charge within the Helmholtz layer is zero, i.e., at the zero point of charge, (pH ZPC), V H is also zero. The flatband potential at the pH ZPC (U ft0) equals the intrinsic Fermi level of the semiconductor, and is the only meaningful flatband potential. Under conditions other than pH ZPC, flatbands are not really flat but contain the band bending induced by the Helmholtz layer. Hence, band edge energies can be calculated from U ft0measurements combined with E g data and an inde-pendent estimate of ΔE F (Nozik 1978).

pH Dependence of band edges at semiconductor/electro-lyte interfaces

For semiconducting metal oxides, U ft varies with pH fol-lowing a linear relation known as the Nernstian relation (But-ler and Ginley 1978; Halouani and Deschavres 1982; Matsumoto et al. 1989; Morrison 1990):

U fb = U fb0 + 2.303R T/(pH ZPC – pH)F(4) where R is the gas constant, T is temperature, and F is the Fara-day constant. At 25 °C and 1 atm, the Nernstian relation leads to a variation of 0.059 V/pH (Fig. 2a). It is generally accepted that the Nernstian dependence indicates that H+ and OH– are potential determining ions (PDI) adsorbed on the solid surface within the Helmholtz layer (Butler and Ginley 1978).

For metal-sulfide semiconductors, the pH dependence of U fb appears more complicated than that for metal-oxides, and has not been as thoroughly studied. One unresolved issue is which ions are PDIs. If ions other than H+ and OH– are PDIs, the specific sorption of these ions can affect V H. For example, Ginley and Butler (1978) showed that dissolved sulfide (H2S and HS–) is a PDI for CdS, whereas our research has shown that dissolved sulfide and dissolved ferrous iron are PDIs for iron sulfides (Bebié et al. 1998; Dekkers and Schoonen 1994). For CdS, the flat band shows a variation in pH dependence from 0 to 59 mV/pH (Ginley and Butler 1978), with the slope depending on the concentrations of metal ions and dissolved sulfide ions. Minoura et al. (1977) reported that U ft varies with crystal face. For FeS2 and ZnS, however, the pH dependence is reported to be consistent with the Nernstian relation (Chen et al. 1991; Fan et al. 1983). We speculate that U ft for metal sul-fide semiconductors may follow the Nernstian pH dependence in aqueous solutions with low concentrations of metal ions and/ or dissolved sulfide, but that the pH dependence will switch to non-Nernstian behavior when specific sorption of metal ions and sulfide becomes important. More research in the surface chemistry of metal sulfides is needed to resolve the apparent inconsistencies highlighted above.

The pH-E H relationship defined by the Nernstian slope is characterized by a constant fugacity of hydrogen, as illustrated in Figure 2b:

log

.

f

E F

T

H

H

2

pH

R

=??

2

2

2303

(5a) or

log f pe

H

2

22

=??

pH

(5b) The constant hydrogen fugacity defined by the Equations 5a or 5b indicates a constant redox state for a linear combination of pH and E H in the Nernstian slope. On a pH-E H diagram, it is the linear combination of E H(or pe) and pH that describes the re-dox state of an aqueous redox system and not E H (or pe) alone; see also Anderson and Crerar (1993). Hence, we believe that the pH dependence of semiconductor flatband potential given by the Nernstian slope is consistent with a constant reducing

F IGURE 2. pH dependence of the conduction band edge and valence band edge of pyrite in an aqueous electrolyte solution described by (a), a pH-Eh diagram, and (b) a pH-log f H

2

diagram. The E C and E V follow the Nernstian relation, which has a variation of 0.059V/pH at 25 °C at 1atm, and lies parallel to the water stability limits.

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES547 (oxidizing) power of the (photo)electrons (holes) in a semi-

conductor. It is noteworthy that the flat-band potentials for

semiconductors are parallel to the stability limits of water in

an E H-pH diagram if they follow the Nernstian behavior. If

log f H

2 is used as the parameter defining the redox condition

of a semiconductor/electrolyte system, as shown in Figure 2b, both the band edges and water stability limits are inde-pendent of pH. Hence, a pH-E H pair can be replaced with a single variable, the fugacity of hydrogen, to define the redox condition of the system.

Temperature and pressure dependence of band edge energies

Because semiconductors can catalyze non-photoreactions as well as photoreactions, their catalytic activity may extend to subsurface geochemical processes, such as in hydrothermal systems. To evaluate the potential of semiconductors as cata-lysts in subsurface environments, it is important to understand the effects of temperature and pressure on the band energies.

In contrast to aqueous redox couples, which often show a considerable temperature dependence for E redox, the electronic structure of a semiconductor undergoes little change with tem-perature and pressure provided that no phase transition occurs. For example, the measured bandgap changes over temperature (d E g/d T) for PbS and ZnO range from +4 × 10–4 eV/K to –9.5 ×10–4 eV/K, respectively (Gonzalez et al. 1995). For most semi-conductors, the variation of the bandgap with pressure is also very small in the pressure ranges of interest. The experimental determined variation rates (d E g/d P) are in the order of 0.01–0.1 eV/GPa (Gonzalez et al. 1995). Therefore, the reducing power of a (photo)electron and the oxidation power of a hole are essentially constant with temperature and pressure, unless the solid itself undergoes a phase transition and changes the electronic structure altogether.

Although temperature and pressure have negligible effect on the bulk band structure of semiconductors in the T-P ranges of interest, they may have a more pronounced effect on inter-facial energetics. As shown in Equation 4 the Nernstian slope is temperature dependent. Perhaps more importantly, the pH ZPC of semiconductors is also temperature dependent. Schoonen (1994) estimated pH ZPC values for several metal oxides up to 350 °C based on extrapolation of low-temperature (<95 °C) experimental data. These calculations indicate that pH ZPC values shift down by 1 to 2 pH units as temperature rises from 25 °C to 200 to 300 °C, but beyond 200 to 300 °C the pH ZPC shifts back to higher values. This general trend was confirmed experimentally for rutile up to 250 °C by Machesky et al. (1994). A 2-pH-unit decrease in pH ZPC between 25 and 250 °C would result in a shift of interfacial band edge energies to a higher energy level (with respect to vacuum) by about 0.25 eV. However, the exact tem-perature dependence of pH ZPC of semiconductors is largely un-known and more experimental investigations are needed.

B AND EDGES OF SEMICONDUCTING OXIDE AND

SULFIDE MINERALS

The conduction band edges and bandgaps for common ox-ide and sulfide semiconducting minerals are given in Tables 1 and 2. This compilation includes data obtained from two dif-ferent methods: photo-electrochemical measurements, and empirical calculation based on electronegativity of constituent atoms. A comparison of experimental and empirically calcu-lated conduction band edges is shown in Figure 3. The trends in the energy positions of band edges for metal oxides and metal sulfides will be discussed below separately. Determination methods

Band edges can be derived experimentally from the flatband potential measurement using various (photo)electrochemical techniques. The classic method for flatband potential determi-nation, which is still considered as the most reliable technique, is the Schottky-Mott method (Nozik 1978). Another common method is the determination of anodic photocurrent onset po-tential by evaluating the photocurrent-potential plot (Arico et al. 1990; Butler 1977). More recently, the photocurrent-volt-age measurements have been extensively used in semiconduc-tor particle systems with electrochemical charge-collection techniques. The advantage of this technique is that the varia-tion of the steady-state photocurrent can be measured as a func-tion of pH. This allows for the estimation of interfacial energetics of the particulate system (Chen et al. 1991; Leland and Bard 1987). It is noteworthy that colloidal particles have a larger bandgap than large crystals due to the quantum size ef-fect, which results in a higher energy position of the conduc-tion band edge (Ward and Bard 1982).

In the last two decades, quantitative quantum mechanical calculations have been carried out for a large number of

semiconductor minerals (Folkerts et al. 1987; Huang and Ching

F IGURE 3. Correlation between the empirically calculated conduction band edge energy and the measured flatband potential at pH ZPC for semiconducting metal oxide and sulfide minerals in absolute vacuum scale. The sources of the experimental data are given in the Tables 1 and 2.

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES

5481993; Lauer et al. 1984; Santoni et al. 1992; Schroer et al. 1993;Temmerman et al. 1993). In principle, band energies can be derived from these calculations. In many studies, however, band edges are presented with the Fermi level as an arbitrary refer-ence point. Hence, although detailed electronic structures of valence and conduction band have been derived, absolute en-ergies of band edges with respect to vacuum cannot be obtained from these calculations. In the few reports in which the abso-T ABLE 1. Absolute electronegativity (x ), band gap (E g ), energy levels of caluculated conduction band edge (E CB ) and flatband potential

at pH ZPC (U ft 0) with respect to Absolute Vacuum Scale (AVS), and measured or estimated pH ZPC for semiconducting metal oxide minerals

Mineral

x E g E CB U ft

0PH ZPC Ref. for Ref. for

eV eV eV eV E g ,U ft 0

pH ZPC

Ag 2O 5.29 1.20–4.6911.20a a

AlTiO 3

5.44 3.60–3.648.23b BaTiO 3 5.12 3.30–4.58–4.219.00c a

Bi 2O 3

6.23 2.80–4.83–4.82 6.20d CdO 5.71 2.20–4.61–4.6211.60c a

CdFe 2O 4

5.83 2.30–4.68–4.897.22c Ce 2O 3

5.20 2.40–4.008.85i CoO 5.69 2.60–4.397.59b

CoTiO 3

5.76 2.25–4.647.41k Cr 2O 3

5.68 3.50–3.938.10b a CuO 5.81 1.70–4.96–4.899.50c a

Cu 2O

5.32 2.20–4.228.53e CuTiO 3

5.81 2.99–4.327.29k FeO 5.53 2.40–4.338.00b

Fe 2O 3

5.88 2.20–4.78–4.698.60c a Fe 3O 4

5.780.10–5.73

6.50f a FeOOH 6.38 2.60–5.089.70g l

FeTiO 3

5.69 2.80–4.29–4.56

6.30a a Ga 2O 3

5.35 4.80–2.958.47b HgO

6.08 1.90–5.13

7.30b a Hg 2Nb 2O 7 6.21 1.80–5.31–5.05 6.25h

Hg 2Ta 2O 7

6.24 1.80–5.34 6.17h In 2O 3

5.28 2.80–3.888.64b KNbO 3 5.29 3.30–3.648.62b

KTaO 3

5.32 3.50–3.57–3.708.55h La 2O 3

5.28 5.50–2.5310.40i m LaTi 2O 7 5.90 4.00–3.907.06b

LiNbO 3

5.52 3.50–3.778.02b LiTaO 3

5.55 4.00–3.557.94b MgTiO 3 5.60 3.70–3.757.81b MnO 5.29 3.60–3.498.61b

MnO 2

5.950.25–5.83 4.60j l MnTiO 3 5.59 3.10–4.047.83b

Nb 2O 5

6.29 3.40–4.59–4.16 6.06h Nd 2O 3

5.21 4.70–2.878.81i NiO 5.75 3.50–4.0010.30b a NiTiO 3 5.79 2.18–4.707.34d PbO 5.42 2.80–4.02–4.468.29d PbFe 12O 19 5.85 2.30–4.70–5.207.17c PdO 5.79 1.00–5.297.34b

Pr 2O 3

5.19 3.90–3.248.87i Sb 2O 3

6.32 3.00–4.82 5.98b

Sm 2O 3

5.26 4.40–3.078.69i SnO 5.69 4.20–3.597.59b SnO 2

6.25 3.50–4.50–4.55 4.30d a

SrTiO 3

4.94 3.40–3.24–3.618.60c a Ta 2O 5

6.33 4.00–4.33–3.89 2.90a a Tb 2O 3 5.33 3.80–3.448.50i

TiO 2

5.81 3.20–4.21–4.16 5.80k a Tl 2O 3

5.35 1.60–4.558.47b V 2O 5

6.10 2.80–4.70–4.84 6.54c

WO 3

6.59 2.70–5.24–5.290.43a a Yb2O 3

5.47 4.90–3.028.15i YFeO 3 5.60 2.60–4.30–4.607.81a ZnO 5.79 3.20–4.19–3.918.80d a

ZnTiO 3

5.80 3.06–4.277.31k ZrO 2 5.91 5.00–3.41–3.08

6.70a a

Notes: a = Butler and Ginley 1978; b = Quarto et al. 1997; c = Nozik 1978; d = Halouani and Deschavers 1982; e = Rodriguez et al. 1998; f = Zhang and Satpathy 1991; g = Brezonik 1993; h = Kung et al. 1977; i = Shelykh et al. 1996; j = Shuey 1975; k = Oosawa et al. 1989; l = Sverjensky 1994,m = Yoon et al. 1979.lute band energies were given, the discrepancies between cal-culated and measured band edges were as large as a few eV (Bullett 1982; Caldas et al. 1984; Fazzio et al. 1984; Tian and Shen 1989).

In a much simpler approach, Butler and Ginley (1978) pro-posed that band edges at the semiconductor/electrolyte inter-face can be predicted from the electronegativity of the semiconductor. This method was based on Equations 4 and 5

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES549

(Fig.1), and the argument that the bulk electronegativity, χ, of a compound is the geometric mean of the electronegativities of the constituent atoms. Hence, for a M a X b compound, the con-duction band edge can be expressed as:

E C = – (χM aχX b)1/(a+b) –1/2 E g + 0.059(pH ZPC – pH) + E0(6) where χM andχX are the absolute electronegativity of the atoms M and X, respectively. Butler and Ginley (1978) were able to demonstrate that E C values for oxide semiconductors calculated using Equation 6 are in good agreement with the experimental values. This method was later applied to other non-oxide semi-conductors, and various electronegativity scales were used for calibration (Halouani and Deschavres 1982; Sculfort and Gautron 1984).

In Tables 1 and 2, we present conduction band edges calcu-lated based on the absolute electronegativities compiled by

T ABLE 2. Absolute electronegativity (x), band gap (E g), energy levels of calculated conduction band edge (E CB) and flatband potential at pH ZPC (U ft0) with respect to Absolute Vacuum Scale(AVS), and measured or estimated pH ZPC for metal sulfide minerals Mineral x E g E CB,U ft0,pH ZPC,Ref. for Ref. for eV eV eV eV E g,U ft0pH ZPC

Ag2S 4.960.92–4.50 2.00n

AgAsS2 5.49 1.95–4.51 2.00n

AgSbS2 5.37 1.72–4.51 2.00n

As2S3 5.83 2.50–4.58 2.00o

CdS 5.18 2.40–3.98–3.88 2.00p

Ce2S3 4.63 2.10–3.59 2.00i

CoS 5.170.00–5.17 2.00q

CoS2 5.490.00–5.49 1.50r hh

CoAsS 5.210.50–4.96 2.00n

CuS 5.270.00–5.27 2.00q

Cu2S 4.99 1.10–4.44 2.00n

CuS2 5.570.00–5.57 2.00r

Cu3AsS4 5.39 1.28–4.75 2.00n

CuFeS2 5.150.35–4.97 1.80s hh

Cu5FeS4 5.05 1.00–4.55 2.00s

CuInS2 4.81 1.50–4.06 2.00t

CuIn5S8 4.72 1.26–4.09 2.00u

Dy2S3 4.78 2.85–3.36 2.00j

FeS 5.020.10–4.97 3.00aa hh

FeS2 5.390.95–4.92–4.96 1.40v hh

Fe3S4 5.180.00–5.18 2.00s

FeAsS 5.110.20–5.01 1.50s hh

Gd2S3 4.84 2.55–3.57 2.00j

HfS2 5.27 1.13–4.71–4.63 2.00s

HgS 5.52 2.00–4.52–4.74 2.00w

HgSb4S8 5.65 1.68–4.81 2.00n

In2S3 4.70 2.00–3.70–3.69 2.00x

La2S3 4.70 2.91–3.25 2.00j

MnS 4.81 3.00–3.31 2.00y

MnS2 5.240.50–4.99 2.00z

MoS2 5.32 1.17–4.73–4.43 2.00p

Nd2S3 4.65 2.70–3.30 2.00j

NiS 5.230.40–5.03 2.00y

NiS2 5.540.30–5.390.60w hh

OsS2 5.74 2.00–4.74 2.00w

PbS 4.920.37–4.74–5.05 1.40d hh

Pb10Ag3Sb11S28 5.29 1.39–4.59 2.00n

Pb2As2S5 5.41 1.39–4.71 2.00n

PbCuSbS3 5.22 1.23–4.61 2.00n

Pb5Sn3Sb2S14 5.270.65–4.95 2.00n

Pr2S3 4.62 2.40–3.43 2.00j

PtS2 6.000.95–5.53 2.00w

Rh2S3 5.36 1.50–4.61 2.00w

RuS2 5.58 1.38–4.89 2.00bb

Sb2S3 5.63 1.72–4.72 2.00cc

Sm2S3 4.69 2.60–3.39 2.00j

SnS 5.17 1.01–4.66 2.00n

SnS2 5.49 2.10–4.44 2.00w

Tb2S3 4.75 2.50–3.51 2.00j

TiS2 5.110.70–4.76 2.00w

TlAsS2 5.06 1.80–4.16 2.00n

WS2 5.54 1.35–4.86 2.00w

ZnS 5.26 3.60–3.46–2.95 1.70dd hh

ZnS2 5.56 2.70–4.21 2.00ee

Zn3In2S6 5.00 2.81–3.59 2.00ff

ZrS2 5.20 1.82–4.29–4.28 2.00gg

Notes: n = Boldish and White 1998; o = Mills et al.1988; p = Sculfort and Gautron, 1984; q = Sugiura et al., 1974; r = Bullett 1982; s = Jaegermann and Tributsch 1988; t = Neff et al. 1985; u = Bicelli 1987; v = Wei and Osseo–Asare 1997; w = Dovgii and Bilen’kiiet al. 1966; x = Becker et al. 1986; y = Freidman and Gubanov 1983; z = Temmerman et al. 1993; aa = Sakkopoulos et al.1984; bb = Hunag and Chen 1988; cc = Efstathiou and Levin 1968; dd = Kanemoto et al. 1992; ee = Lauer et al. 1984; ff = Poulios and Papadopoulos 1990; gg = Chamelra et al. 1987; hh = Beibie et al. 1996.

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES 550

Pearson (1988). The absolute electronegativities for rare-earth elements are calculated from the ionization energy and elec-tron affinity data by Huheey (1978) using Equation 1. Because the zero-energy point of the absolute electronegativity scale is that of electrons at rest in a vacuum rather than an artificial reference point (such as Pauling’s scale), the calculated band edges are expressed in the AVS scale directly. The energy posi-tions of band edges in the electrochemical scale can be con-verted from the values in A VS scale using:

E(NHE) = –E(AVS) – 4.50(7) As indicated in the Equation 6, it is crucial to know pH ZPC to estimate interfacial band edge energies. For a number of oxide minerals, pH ZPC values have been obtained experimentally (But-ler and Ginley 1978; Davis and Kent 1990). For those oxides experimental values of pH ZPC were not available, pH ZPC values are estimated using the method developed by Butler and Ginley (1978). For metal sulfide minerals, we use data reported by Dekkers et al. (1994) and Bebié et al. (1998). Because pH ZPC values for most metal sulfides are at or below pH = 2 in solu-tions where no metal ions and/or dissolved sulfide are added, a value of 2.0 was used in the calculation when sulfides experi-mental values of pH ZPC were not available. The flatband poten-tials at pH ZPC (U0fb) shown in Tables 1 and 2 are calculated from the measured flatband potentials at various experimental pH values using the Nernstian relation given by Equation 4. The calculated band edges are generally within 0.5 eV of U fb0 val-ues, as shown in the Figure 3.

Systematics of band edge positions of metal oxide semiconductors

Figure 4 shows the conduction and valence band edge posi-tions at pH 0 obtained from empirical calculation for a number of semiconducting oxides which may be important in aqueous geochemical systems. The correlation between band edges and electronic structures will be discussed briefly below.

For non-transition metal oxides (ZnO, PbO, CdO, SnO2,etc.), the bottom of the conduction band is primarily from the metal s orbitals, whereas the top of the valence band is derived primarily from oxygen 2p orbitals. The top of the valence band of most oxides of transition metals with low d electron occu-pancy (such as TiO2, ZrO2, WO3) is also derived from oxygen 2p orbitals, but the conduction band edge for those oxides is generally derived from metal d orbitals (Shuey 1975). These minerals usually have large bandgaps. Their valence band edges are situated at energy levels close to the absolute electronega-tivity of oxygen (–7.54 eV), and much lower than the oxida-tion potential of water (redox potential of O2/H2O couple). The conduction band edges of these minerals are close to the re-duction potential of water (redox potential of H2/H+ couple). Hence, electrons in the conduction band and holes in the va-lence band of these minerals are very high in reducing and oxi-dizing power, respectively.

For oxides of transition metals with high d electron occu-pancy, metal d states are present in both valence band edges and conduction band edges. As a result, the band structures become complicated and bandgaps are generally smaller. In Fe2O3, for example, the top of the valence band is the occupied e g doublets of the Fe 3d orbital with a strong hybridization of the O 2p or-bital, whereas the bottom of the conduction band is the Fe t2g orbitals with opposite spin. For Cu2O, the top of the valence band is derived from the Cu 3d orbital, whereas the bottom of conduction band is derived from the Cu 4s orbitals. In MnO2, the top of the valence band is the t2g triplet of Mn d orbitals, whereas the bottom of the conduction band is the e g doublet of Mn d orbitals.

Fe3O4 is a temperature-dependent Mott-semiconductor/con-ductor. In Fe3O4, there are separate Fe 3d sub-bands for octa-hedral and tetrahedral atoms, and each degenerates to t2g and e g orbitals and has two possible spins. Half of these bands are filled and the 3d electrons could be localized or delocalized depending on the spin-polarization. Below the Verwey transi-tion temperature (120 K), Fe3O4 is a semiconductor when the antiferromagnetism ordering splits the partially filled d

bands

F IGURE 4. Calculated energy positions of conduction band edges and valence band edges at pH 0 for selected metal oxide semiconductors. The bottom of open squares represent conduction band edges, and the top of solid squares represent valence band edges. The solid lines indicate water stability limits.

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES 551

into filled and empty sub-bands with a bandgap of 0.1 eV . How-ever, above the Verwey temperature, Fe 3O 4 is a conductor of low electron mobility (Zhang and Satpathy 1991).Systematics of band edge positions of metal sulfide semiconductors

The electronic structures of sulfide minerals are described in detail by Vaughan and Craig (1978). The relationship be-tween the energy position of band edges and band structures,especially the effect of d electrons of transition metals, is briefly discussed below.

Figure 5 shows the calculated band edge positions of com-mon metal sulfides. The top of the valence band of non-transi-tion metal sulfides is primarily derived from S 3p orbitals,whereas the bottom of the conduction band is mainly derived from metal s orbitals. Both conduction and valence band edges of these sulfides are generally higher in energy than those of oxides. Hence, electrons in the conduction band of these min-erals are higher in reducing power than in their oxide counter-parts, whereas the holes in the valence band are less oxidizing.The valence band edges of sulfides deviate substantially from the absolute electronegativity of sulfur (–6.22 eV) for some of the minerals, perhaps due to the more covalent nature of sul-fide minerals than that of oxide minerals.

As in the case of oxide minerals, when more metal d states are present near the valence and conduction band edges, band gaps are substantially narrowed, and band structure becomes complicated. Exceptions to this general pattern are the Mn sul-fides (both MnS and MnS 2) where Mn has a half-filled d shell (d 5). In MnS, the Mn d orbitals are split into two groups of opposite spin. The top of the valence band is the filled e g or-bital of majority spin, and the bottom of the conduction band derived from the empty t 2g orbital of minority spin, and its measured bandgap is about 3.0 eV (Freidman and Gubanov 1983). From FeS to CuS, additional d electrons gradually fill in the minority-spin t 2g orbitals. Because of their partially filled d bands, CoS and CuS are conductors. FeS and NiS are, how-

ever, temperature-dependent Mott-semiconductors, with tran-sition temperatures of 573 K and 264 K, respectively (Sakkopoulos et al. 1984, 1986). Below the transition tempera-ture, FeS and NiS are semiconductors with a bandgap of 0.2eV and 0.4 eV, respectively.

Among transition-metal disulfides, FeS 2 is a semiconduc-tor, MnS 2 and NiS 2 are Mott-semiconductors, whereas CoS 2 and CuS 2 are conductors. Band structures of these minerals have been discussed in detail by Vaughan and Craig (1978), Bullett (1982), Lauer et al. (1984), and Folkerts (1987). In MnS 2, d orbitals split into two spin-groups with a high-spin configura-tion. The top of the valence band is the occupied t 2g orbitals with majority spin, and the bottom of the valence band is unoc-cupied e g orbitals with minority spin, separated by a 1.7 eV gap. In FeS 2, d electrons have a low-spin configuration. The full t 2g orbitals make up the top of the valence band, whereas the empty e g * orbitals make up the conduction band. Pyrite has a bandgap of 0.95 eV. In NiS 2, the top of the valence band is derived from the full minority spin t 2g orbitals, whereas the bottom of the conduction band is derived from the empty mi-nority-spin e g * orbitals, separated by a bandgap of about 0.27eV . Although FeAsS has a crystal structure derived from the pyrite structure, its electronic structure is remarkably differ-ent. In FeAsS, the t 2g orbitals are further split. The empty split-off t 2g sub-band of higher energy constitutes the bottom of the conduction band, whereas other occupied non-bonding orbit-als serve as the top of the valence band, separated by a bandgap of 0.2 (Shuey 1975; Vaughan and Craig 1978).

B AND EDGE ENERGIES AND INTERFACIAL ELECTRON

TRANSFER

Interfacial electron transfer is a complicated process con-strained by a variety of thermodynamic and kinetic factors. The sorbed aqueous species will interact with electronic surface states, which may be different in energy from the bulk band edges. One may expect that the interaction between an aque-

ous redox species and a mineral surface can be predicted only

F IGURE 5. Calculated energy positions of conduction band edges and valence band edges at pH 0 for selected metal sulfide semiconductors.The bottom of open squares represent conduction band edges, and the top of solid squares represent valence band edges. The solid lines indicate water stability limits.

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES 552

if the surface electronic state is taken into account. Experimental

studies (Inoue et al. 1979; Kanemoto et al. 1992, 1996) and our

work (Xu and Schoonen 1995; Xu et al. 1996, 1999) suggest,

however, that the electron transfer between semiconductors and

aqueous redox species only occurs at a semiconductor/electro-

lyte interface where two orbitals, one belonging to the semi-

conductor and one belonging to aqueous species, are of

approximately the same energy. This suggests that thermody-

namic properties of the semiconductor and the electrolyte, i.e.,

the relative energies at the semiconductor/electrolyte interface,

are the fundamental constraints that dictate the interfacial elec-

tron transfer processes. The following section reviews three

types (direct electron injection, photosensitized electron injec-

tion, and the photo activation of a semiconductor) from the

point of view of interfacial energetics. This simplified treat-

ment ignores the fact that the transfers must occur between the

frontier orbitals of sorbate and semiconductors, which may raise

activation barriers, and does not account for changes in free

energy due to chemisorption, which may shift the E0oc and E0unoc

with respect to the values for those species in solution. In fact,

the formation of a chemisorbed complex is also a key to cata-

lytic activity and selectivity. To go beyond our crude approach

presented below, however, one would have to obtain a wealth

of information of the exact reaction mechanism and associated

energetics for each reaction of interest.

Direct electron injection into the conduction band of a

semiconductor

If the redox potential of an aqueous redox couple is higher

in energy than the conduction band edge of a mineral, the di-

rect electron transfer from the aqueous electron donor into the

conduction band of the mineral can proceed because such a

transfer is energetically favorable. For example, Freund (1969)

found that Cr2+, Eu2+, V2+, and Ti3+(which all have E0 higher

than –4.4 eV A VS) can inject electrons into the ZnO conduction

band (E C = –4.3 eV A VS at pH 7), but Co2+, Fe2+, and Mn2+ cannot

(which all have E0lower than –4.4 eV AVS). Kohl and Bard also

showed that the direct electron injection into the conduction

band of a semiconductor does not occur when the redox poten-

tial of the redox couple is appreciably below the flatband po-

tential of the solid (Kohl and Bard 1977).

Semiconductor particles in an electrolyte solution may be-

have as a short-circuited electrochemical cell, with both ca-

thodic and anodic reactions occurring on the same particle.

Because of the delocalized nature of electrons in the conduc-

tion band of a semiconductor, electrons injected from an aque-

ous donor can be transferred to an aqueous electron acceptor at

a different site (Fig. 6a). This process may facilitate the elec-tron transfer from aqueous electron donors to aqueous electron acceptors when direct electron transfer is inhibited by symme-try mismatch of their frontier orbitals. For example, pyrite and galena, as well as Ni- and Cu-doped sphalerite can catalyze the reaction between thiosulfate and dissolved molecular oxygen to tetrathionate (E0 = –4.6eV A VS), a redox reaction that does not proceed at any significant rate in homogeneous aqueous sys-tems (Xu and Schoonen 1995; Xu et al. 1996). Although the absolute energy (i.e., reducing power) of electrons does not increase when electrons are injected from an aqueous electron

F IGURE 6. Electron transfer processes at a semiconductor/ electrolyte interface. (a) direct injection of electrons from an aqueous electron donor into the conduction band of a semiconductor and the subsequent transfer of the electron to an aqueous electron acceptor. (b) indirect electron-injection from an aqueous electron donor sorbed on the semiconductor surface into the conduction band through a photo-sensitizer excited with sub-band irradiation, and the subsequent transfer of the electron to an aqueous electron acceptor; (c) photo-induced formation of electron-hole pair in a semiconductor, and the subsequent reactions of the conduction electron and valence hole with an aqueous electron acceptor and an

electron acceptor, respectively.

donor to the conduction band of a semiconductor, this hetero-geneous electron transfer process does provide an alternative pathway for aqueous redox reactions. It is noteworthy that the semiconductor does not need to be illuminated for direct elec-tron injection to proceed. Hence, the direct injection of an elec-tron into the conduction band of a semiconductor may be an important catalytic mechanism beneath the photic zone in aquatic systems.

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES553

Photo-sensitized electron injection into the conduction band of a semiconductor

For direct electron injection into a semiconductor to occur, the redox potential of the electron donor must have higher en-ergy than the conduction band of the semiconductor. For semi-conductors such as ZnS, TiO2, ZrO2, SnO2, where conduction band energy is very high, it requires an electron donor with redox potential at even higher energy. This requirement restricts the utilization of these minerals as catalysts for aqueous redox reactions via a direct electron injection mechanism. However, electrons can be injected into conduction bands of these min-erals through the excitation of an aqueous photo-sensitizer sorbed on the semiconductor (Fig. 6b). In the photo-sensitized electron injection, an electron in the highest occupied molecu-lar orbital (HOMO) of the sensitizer is excited to its lowest unoccupied molecular orbital (LUMO) with a sub-bandgap ir-radiation (i.e., photon energy less than E g of semiconductor), and is subsequently injected into the conduction band of the semiconductor. Concurrently, an electron is transferred from the aqueous electron donor to the hole created in the HOMO of the sensitizer by the irradiation. Many factors are important to the photo-sensitized electron injection, such as the type of the chemical bond that connects the semiconductor and the sensi-tizer, and competitive sorption. The most fundamental prop-erty controlling this process, however, is the relative energies at the semiconductor/sensitizer/electrolyte interface. To inject an electron to the conduction band of the semiconductor, the LUMO of the sensitizer must have an energy higher than the conduction band edge of the semiconductor. To provide the driving force for the hole transfer, the energy level of HOMO of the sensitizer must be lower than the redox potential of the electron donor in the solution (Waite 1990; Zaban et al. 1998).

Photo-sensitized electron injection is an important mecha-nism in the photic zone of aquatic systems. For example, pho-tosensitized interfacial electron transfer involving sediments coated with a humic substance is critical for the reduction of metallic pollutants (Selli et al. 1996). The photoreductive dis-solution of ferric (hydr)oxide sensitized by surface complexes, such as Fe(III)-oxalate or Fe(III)-EDTA, is an important mecha-nism in controlling iron concentration in natural environments (Faust and Hoigne 1987; Stumm and Morgan 1995; Stumm and Sulzberger 1992; Sulzberger 1990).

Photo-induced electron transfer at the semiconductor/ electrolyte interface

The irradiation of a semiconductor with ultra-bandgap en-ergy results in the promotion of an electron from the valence band to the conduction band, with the concomitant generation of a hole in the valence band. The photo-excited electrons and holes can then react with aqueous electron acceptor and donor, respectively (Fig. 6c). If the ΔG R0 of the overall reaction be-tween the electron acceptor and donor is negative, the reaction is a photocatalytic proccess. Alternatively, if the ΔG R0 is posi-tive, the reaction is a photosynthetic process. The photocataly-sis and photosynthses have been the subject of numerous investigations, and research in this area is growing rapidly. To illustrate the role of interfacial energetics in controlling the elec-tron transfer in these processes, we briefly mention two well-investigated processes: (1) photoreduction of CO2 and (2) pho-todecomposition of H2O.

Semiconductor-catalyzed CO2 photoreduction is a poten-tially important process in the prebiotic synthesis of organic compounds and the origin of life as well as in efforts to counter the greenhouse effect. The CO2 photoreduction to organic com-pounds has been studied with suspensions of various semicon-ductor particles, including WO3, TiO2, SnO2, CdS, GaP, SiC, and ZnS (Inoue et al. 1979; Kanemoto et al. 1992). To reduce CO2, one needs a semiconductor with a conduction band edge higher than the CO2 reduction potential (E0 = –3.9 eV for the redox couple of HCOOH/H2CO3 at pH 5). ZnS, SiC, GaP, CdS can effectively reduce CO2, whereas WO3, TiO2, and ZnO can-not (for E C values, see Tables 1 and 2, and Figs. 4 and 5).

Similar constraints on the effectiveness of semiconductor ca-talysis by conduction-band energy have been shown for H2O pho-todecomposition. For example, the E C of rutile is lower than the redox potential of H2/H2O (E0 = –4.5 eV V AS at pH 0), hence rutile cannot photo-reduce H2O to H2. In an early photoelectrolysis study of H2O on the rutile electrode, a 0.2 V bias was added to generate H2 (Inoue et al. 1979). Because of the quantum size effect, col-loidal TiO2 particles have a higher E C value than the H2O/H2 redox couple. Consequently, no bias is required for H2O pho-todecomposition in the presence of a colloidal TiO2 suspen-sion (Gr?tzel 1988, 1989). Semiconductors with conduction band edges higher than that of the water reduction potential, such as SrTiO3 and other ternary titanates (Oosawa et al. 1989) and CdS (Aspnes and Heller 1983; Darwent 1981), have been reported to generate H2 via water photoreduction.

Sphalerite has a very high conduction band edge energy (Table 1 and Fig. 5), hence it is to be expected that sphalerite would photochemically reduce water to H2. This photoreduc-tion process has been demonstrated by Reber and Meier (1984) and Kanemoto et al. (1992). However, Yanaginda et al. (1990) reported that defect-free quantized ZnS particles do not cata-lyze the photoreduction of water to H2, but catalyze energeti-cally less-favorable reactions such as reduction of aldehydes and aliphatic ketons. Yanaginda et al. (1990) attributed the lack of H2 production to the large energy gap between the sphalerite E C and the potential of water reduction. Although the rates of electron transfer reactions generally increase with increase in the driving force (i.e., the energy difference between electron donor and acceptor levels), a large difference in energy be-tween donor and acceptor levels results in a slow electron trans-fer. This phenomenon has been described as the “inverted region effect” (Miller 1987). The optimum energetic relation for elec-tron transfer at a semiconductor/electrolyte interface has not been investigated systematically. Yanaginda et al. (1990) pointed out that only the surface states formed within the bandgap of ZnS serve as active sites for the H2 evolution. Li and Morrison (1985) also showed that one of the mechanisms for high exoenergic electron transfer involves moving electrons to aqueous oxidant through states at dislocations.

At valence bands, photo-induced holes can take part in two different types of reactions. First, if the redox potential of the aqueous couple is higher in energy than the valence band edge, electrons will be transferred from the aqueous donor to fill the hole in the valence band, thereby oxidizing the aqueous elec-

XU AND SCHOONEN: SEMICONDUCTING OXIDES AND SULFIDES 554

tron donor. As a photo-catalyst, semiconducting minerals can photo-oxidize a wide range of compounds. Some geochemi-cally and environmentally important reactions include photo-oxidation of phenol (Augugliaro et al. 1988), PCBs (Pelizzetti et al. 1988), sulfur-containing organic compounds (Davis and Huang 1991; Spikes 1981), insecticides (Harada et al. 1990) and nitrogen-containing organic compounds (Ferry and Glaze 1998; Low et al. 1991; Takeda and Fujiwara 1996). Second, if the standard potential for anodic decomposition of the semi-conductor is higher in energy than the valence band edge, the hole can also cause oxidation of the semiconductor. In aque-ous solutions, the photo-electrochemical stability of a semi-conductor is determined by the standard potential for anodic decomposition relative to the oxidation potential of water. Most metal oxide semiconductors are kinetically stable against photo-oxidation in water (Figs. 4 and 5), whereas most sulfide semi-conductors are unstable. By introducing a sacrificial electron-donor (i.e., an electron donor with a redox potential higher than the anodic decomposition potential of a semicon-ductor), a photo-electrochemically unstable semiconductor can be stabilized kinetically. For example, reducing agents such as HS–, SO32–, and S2O32– have been used to stabilize sulfide semi-conductors in aqueous solutions (Inoue et al. 1979; Minoura et al., 1977). In this case, photooxidation of the semiconductor is eliminated because the valence band process is dominated by the oxidation of the sacrificial aqueous electron donor.

These two types of photoelectron transfer processes associ-ated with a valence band indicate that the energy position of valence band edges is crucial for determining the activity and stability of a semiconducting mineral in aqueous geochemical environments. For example, under anaerobic conditions, aque-ous sulfur oxyanion species can be generated from the photo-corrosion of a semiconducting sulfide mineral without the involvement of a strong oxidant such as molecular oxygen. The significance of this process in controlling aqueous sulfur spe-ciation, and consequently the redox state, in the prebiotic oceans is still an unexplored subject.

A CKNOWLEDGMENTS

This paper results from the Ph.D. studies by Y.X. at SUNY-Stony Brook. This work was supported by grants from NSF-EAR and NASA-Exobiology to M.S.. We thank R. Reeder, J. Parise, and D. Strongin, A. Suero, and R. Penfield for comments on an earlier version, D. Vaughan and an anonymous reviewer for their thorough reviews, and M. Fleet for his editorial handling of the manu-script.

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(一)经营环境 1. 国际经营环境 整个国际环境经济萎靡，大部分垮地区、垮国界的投资在衰退。小型家用电子产品国际贸易有上升趋势 2. 国内经营环境 一线城市不断成熟，二线城市不断崛起，国家经济重点开发区快速发展 (二) 市场需求 1. 行业环境：随着人们生活的稳定，品味的提高，消费者对美的追求，对人体秤也越来越受女性朋友的追捧，随着人们健康的要求也越来越高，厨房秤也随之走进千家万户。市场需求在不断提高，行业处于快速发展期。

市场推广年度工作计划范文

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牌地位，再根据流通类产品的推出，再将推广的重心向目标消费群体转移，完成专业领域的强势品牌向大众化品牌的转化。 · 推行体验营销，围绕核心竞争力以服务为舞台、以商品为道具、以消费者为中心，创造、设计一系列能够使消费者参与、值得消费者回忆的活动（路演）模式，让消费者亲自体验安尚照明带来的利益； · 以行业内、跨行业、渠道内、渠道外的联合促销为主要推广方式，化的利用传播资源，形成一个立体的装饰、建筑消费整体。 【一、推广目标】 · 销售任务目标：完成年度销售任务6125万； · 广告宣传目标：在推广年度内确立行业内专业品牌地位，在行业内具有一定知名度； · 渠道目标：完成全国布点任务，在全国主要一、二类市场建立销售网点；并在推广年度末在全国范围内建立100家以上专卖店；全国隐性渠道体系的建立。 · 管理目标：在推广年度内建立完善的推广策划、执行、反馈、评估体系； · 产品目标：完善产品线；确立在市场竞争力的核心产品群。 【二、推广目标受众】 · 推荐作用类：设计院、监理公司、建委； · 目标客户类：建设单位、房地产公司、装修公司等 · 渠道成员：经销商、导购人员、员工等 【三、重点推广区域】 全国发达地级市以上市场

城市轨道交通再生制动能量回收系统研究

华东理工大学 毕业设计（论文） 题目城市轨道交通再生制动能量 回收系统研究 学院华东理工大学 专业电气自动化 年级 2016 学号 26140118 姓名 导师 定稿日期： 2016年 11月12 日

摘要 城市轨道交通作为一种运量大、速度快、污染少、舒适性好的交通工具，很有力的缓解大中型城市乘车难、环境污染及交通拥堵等难题。近年来我国着力发展城市轻轨和地铁，本文主要以地铁作为研究对象。城市轨道交通站间距离短、运行密度高，机车频繁制动吋产生相当可观的再生能量，将产生的能量得以利用，不仅节约能源、保护环境同时降低电压利于机车安全运行。再生制动产生的能量得以利用是本文研究的重点，提出逆变电阻混合型再生制动能量吸收方案。本课题以建立地铁再生制动及能量吸收仿真平台为目的，利用仿真软件建立机车运行制动模型及混合型能量吸收模型。首先，分析和总结几种城市轨道交通车辆制动方案的优缺点，重点研究馈能型再生制动方案的基本原理及主要技术问题，提出逆变电阻混合型再生制动能量吸收方案。然后基于电阻制动原理，结合逆变并网电阻制动方案进行建模、仿真分析，并对再生制动产生功率及电流进行粗略的计算。 关键词：再生制动；逆变并网；电阻制动 Abstract

As a large capacity, fast speed, less pollution and comfortable transportation, urban rail transit effectively alleviate the transportation pressure of the large and medium-sized city, environmental pollution and traffic congestion . In recent years, China began to develop the light rail transit and subway. The subway stations has shorter distance and locomotive has haig density running. During locomotive frequently braking, it produced considerable regeneration energy. Reasonable utilization of the regeneration energy not only save energy, protect environment but also reduce the regeneration energy not only save energy, protect environment but also reduce the voltage grade for the locomotive’s safety operation. This paper is the focus on utilization of the regeneration energy, and The inverter-resistance hybrid method is propose. This topic is purposed to build Metreo regenerative braking and inverter-resistance hybrid energy absorption model by simulation software. Firstly, the urban rail transit power supply system has been introduced. Several vehicle braking scheme has been summarized and analyzed for their advantages and disadvantages. The inverter-resistance hybrid of regenerative braking energy absorption solution has been purposed. Secondly, combined with inver and resistance braking scheme, the model was built analyze and the power and current ofregenerative braking was computd.

2020市场部年度营销工作计划

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和资源共享购物广场在营销策划上逐渐向以服务为轴心的商业综合体转化;将原来的美陈的投资大量压缩全部采用租赁的模式。他们的策划方案深受启发很值得我们学习和借鉴。这一点我们市场部已经开始学习和贯彻《商业 4t 营销理论》。把我们原来的供应商体系重新做了梳理，引进大连几家专业从事展览器材及展品租赁的供应商，这样我们将大大节省了 2020 年商业美陈的投入。并随时关注大连万达的发展动向，即时做出调整。购物广场的壮大，离不开新老顾客不间断的物质资助。不断把潜在顾客变为顾客，把顾客变为老顾客、忠实顾客，也将是购物广场发展的必由之路。因此，发展会员，推行会员卡，同时不断通过相关活动把顾客提升为沈阳大悦城家族的一分子、一部分，应是2020年战略规划之一，而会员卡也将在下一年的各个活动中具体体现和运用起来。 二、广告公关 我们做广告的目的，就是第一在消费者心目中树立良好、牢固的企业形象，提高美誉度和认同度;其二就是借助广告媒体对商业信息进行有效传递，提升实效性。两相结合，才是相对完善的广告宣传。 长期以来，我们的广告媒体主要是以电视字幕广告为主流媒体，从实效性来看，的确具有一定的效果，但是作为主流媒体，欠缺的是无法将形象树立在市民心中，而对于现代广告营销而言，电视字幕、短信等广告媒体也只是起到发布信息的作用，并没有完全发挥出“广告形象宣传”的作用。在 2020 年，首要的任务则是根据以往收集来的各广告公司、广告媒体进行深入分析，确定出着实适合我们企业的主流媒体作为宣传平台，并根据该媒体特点制定长期宣传战略，使其切实为我们服务，达到真正广告宣传的目的。其次，在依托主流媒体进行形象宣传的同时，尽可能多地通过各种方式增加社会影响力，如制造新闻看点、发展大型文化主题巡展等

市场推广年度工作计划(标准版)

市场推广年度工作计划(标准 版) Through the work plan, you can make a plan for future work and work out a detailed plan; the work plan function greatly improves work efficiency. ( 工作计划) 部门：_______________________ 姓名：_______________________ 日期：_______________________ 本文档文字可以自由修改

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额的前列，再将区域市场的营销模式总结分析，快速在全国进行推广，完成全国市场的战略布局； ·在市场推广的产品规划中，我们拟采用单个特色化产品、优势（成本优势或技术优势等）产品的重点推广带动整个产品群的提升，确定其在照明行业某一领域的补缺和领导地位，进而通过一系列的市场运作，影响、带动其它相关产品群的提升； ·在市场推广的策略组合中，我们拟希望通过突破传统的照明行业推广手段，选择性的导入其它行业的成功推广模式，以品牌主题推广影响市场推广，以市场推广加速品牌影响，并根据不同时期内市场情况通过组织数次“海陆空”式的纵深立体化战役推广，分步骤的实现营销推广目标。 ·在推广的目标受众选择上我们前期主要集中在行业内的专业人士以及渠道经销商、工程建筑商、承包商等，逐步建立在行业内的专业品牌地位，再根据流通类产品的推出，再将推广的重心向目标消费群体转移，完成专业领域的强势品牌向大众化品牌的转化。

列车再生制动能量回收的方法及分析

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2019年市场推广工作计划模板

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