J. Phys. Chem. 1993,97, 11081-1 1086
11081
Spectroscopic Determination of the Flatband Potential of Transparent Nanocrystalline ZnO Films Gareth Redmond, Angela O'Keeffe, Carol Burgess, Ciara MacHale, and Donald Fitzmaurice' Department of Chemistry, University College Dublin, Dublin 4, Ireland Received: June 22, 1993; In Final Form: August 1 I, 1993'
Ethanolic ZnO dispersions have been characterized by optical absorption spectroscopy, transmission electron microscopy, and X-ray diffraction spectroscopy. Freshly prepared dispersions contain spherical crystallites (hexagonal wurtzite), having an average diameter of 2 nm, and show confinement effects. Dispersions aged at room temperature for 5 days contain spherical crystallites, having an average diameter of 13 nm, and show no confinement effects. Transparent nanocrystalline films (thickness 4 pm) were formed on a conducting glass (SnOz) substrate by sintering 13-nm crystallites in air at 450 OC for 3 h. Incorporation in an electrochemical cell, as the working electrode, permits potentiostatic control of the Fermi level within these films. On applying a potential more negative than the flatband potential, electrons accumulate in the ZnO conduction band. No absorbance which could be assigned to free conduction band electrons was observed between 300 and 800 nm. Charge carrier behavior was monitored by measuring the Burstein shift a t wavelengths shorter than 385 nm. The potential at which a Burstein shift of a given magnitude was observed exhibits the expected Nernstian shift of 0.06V per pH unit for a metal oxide semiconductor in an aqueous electrolyte solution. Calculation of the flatband potential was possible from the measured relationship between the Burstein shift and applied potential at several different pHs.
Introduction Transparent nanocrystalline filmsof a range of semiconductors have been prepared.l-4 If the substrate used is conducting glass, a film may form the working electrode of an electrochemicalcell, and potentiostatic control of the Fermi level within these films is p o ~ s i b l e . ~ To , ~ , ~date, our work has employed transparent nanocrystalline Ti02 films (4-pm thickness) supported on a fluorine-doped SnOzglass substrate.611 On applying a potential more negative than the flatband potential (Vm),electrons are accumulated in the conduction band. The optical cross section of these electrons has been calculated. Determination of both the energy and number of free carriers present at the electrode electrolytesolution interface was therefore possible. Vmfor these polycrystalline electrodes has also been determined.8 We note that determination of Vm by measurement of the space charge differential capacity is generally not possible for polycrystalline semiconductor electrodes, mainly due to the high concentration of surface defects.8J0Jz Preparation of transparent polycrystallinesemiconductor films of Ti02 has also permitted accumulation of free electrons at the electrodeelectrolyte solution interface to be monitored spectroscopically in real time.' Simultaneous monitoring of those spectroscopic changes assigned to reduction of redox species at the above interface has also been possible. Other work directed toward identifying intrinsic and extrinsicintraband states in Ti02 has been described in detail elsewhere.11 Briefly, however, the absorbance change assigned to accumulation of free carriers is monitored in real time following application of a potential step. If there exists a population of vacant intraband sites at which an electron may be localized, the overall rate constant for charge accumulation will be determined to a measurable extent by the rate of electron trapping at these sites. We have been seeking to extend this work to other semiconductors. A recent publication,describinga convenient preparation for highly confined ZnO crystallites,has encouraged us to prepare transparent nanocrystalline films of this semiconductor.13 Potentiostatic control of the Fermi level within these films is possible following their incorporation in an electrochemical cell. On applying a potential more negative than Vmelectrons accumulate Abstract published in Aduance ACS Absrracts, October 1, 1993.
n n 2 ? - ~ 4 1 9 13 m 7 - 1 1081 $04 nn in
in the conduction band. No absorbance which could be assigned to accumulated electrons was detected. Behavior of these charge carriers is therefore monitored by studying the associated Burstein shift. Calculation of the flatband potential has proved possible from the observed relationship between the measured Burstein shift and the applied potential at different pHs. The workoutlined above permits the number and energy of free electrons present at the electrode-electrolyte solution interface to be known. This in turn makes possible a fuller understanding of Faradaic processes occurring at such an electrode. Experimental Section Preparation of Transparent Nanocrystalline ZnO Films. Ethanolic dispersions of ZnO, having an average crystallite diameter of about 2 nm, were prepared following the method of Spanhel and Ander~0n.I~ Reagents used were the following: zinc acetate (Aldrich),lithium hydroxide monohydrate (Aldrich),and absolute ethanol (Merck). Dispersions stored for 1 week at 0 OC in a closed flask showed no evidenceof aging which could be detected by UV-vis absorptionspectroscopy. Experimentsfor which results are reported, unless otherwise stated, employed films consisting of a 4-pm-thick layer ZnO nanocrystallite supported by a 0.5pm-thick layer of fluorine-doped SnOz on glass. A ZnO dispersion, aged at room temperature for 5 days and concentrated to 140 g/L ZnO, was used to prepare these films. The deposited crystallites were spherical and had an average diameter of 13 nm. Finally, films were activated by firing for 3 h in air at 450 "C immediately prior to use. We note that Hotchandani and Kamat have also recently described preparation, by spin coating, of nanocrystalline ZnO films on a conducting glass substrate.4 The above films formed the working electrode (2.0-cmZsurface area) of a three-electrode single-compartment cell, the counter electrode being platinum and the reference electrode a saturated 'calomel electrode (SCE). The electrolyte solution was 0.2 M LiC104. The pH was adjusted using HC104 and KOH. All potentials are reported against SCE. Potential control was provided by a Thompson Electrochem Ministat precision potentiostat. The above cell was incorporated into the sample compartment of a UV/vis spectrophotometer. Electrolyte solutions were prepared from distilled-deionizedwater using lithium perchlorate as supplied by Aldrich. 63 1001 A m p ~ i r ~Phrmical ~ n C--;ptv
Redmond et al.
11082 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
B
300
Wavelength (nm)
400
300
b
1 hr
I I
$
500
700
Wavelength (nm)
0.1 1
1
1:;1
v
-0.4!
.
300
Potential -1.45 V (SCE)
400
.
500
.
'
600
Wavelength (nm) 1 hr
H
120 hrs
'
40nm' 40 nm Figure 1. (a) Absorbance spectra, (b) X-ray diffraction spectra, and (c)
transmissionelectron micrographs of a ZnO colloidal dispersion immediately following preparation and following 5 days aging at room temperature.
Characterization Techniques. All UV/vis absorption spectra were recorded using a Hewlett Packard 8452A diode array spectrophotometer. Spectra of ZnO dispersions were measured in a 0.50-cm optical path length quartz cell, with 0.25 mL of the sample dispersion being made up to 1.50 mL in ethanol. Transmission electron micrographs (TEMs) were obtained using a JEOL 2000FX TEMSCAN from ZnO nanocrystallitesmounted on Formvar-coated copper grids. X-ray diffraction (XRD) spectra were obtained using a Philips PW system from powder ZnO samples.
ResultS Spectroscopy of Ethanolic Dispersions of ZnO Crystallites. Crystallites of ZnO, formed at 0 "C, have an average diameter of 2 nm as determined by TEM. (Those formed at room temperature have an average diameter of 3 nm.) The corresponding XRD spectrum exhibits the broadened peaks expected for hexagonal wurtzite crystallites of this domain size.13J4 The onset for optical absorption at about 350 nm is significantlyblueshifted with respect to bulk ZnO, Le., 385 nm at 300 K.15 Dispersions stored at room temperature for 5 days age and have an average crystallite diameter of 12 nm as determined by TEM. Analysis of the corresponding XRD spectrum gives a domain size of 13 nm. The agreement of values obtained from XRD and TEM measurements indicates broadening in the XRD spectrum is principally a consequence of smaller domain size and not due to lattice defects.16 Crystallite growth is accompanied by a red shift in the onset for optical absorption to the value expected for the bulk material. The UV/vis, XRD, and TEM data referred to above are shown in Figure 1.
Figure 2. (a) Absorbance spectra (pH 12.0) of a transparent nano-
crystallinefilm, prepared from 2-nm ZnO sphericalcrystallites, (1) prior to and (2) following activationby firing in air at 450 O C for 3 h. (b) The Burstein shift observed for films (1) and (2) in (a) at -1.45 V (SCE).
Spectroscopy of Nanocrystalline ZnO Films. The UV/vis spectroscopy of films formed by deposition of a 4-pm layer of ZnO crystallites is similar to that observed for the corresponding ethanolic dispersions. Films prepared from freshly formed nanocrystallites exhibit a highly confined spectrum. Firing of these films, in air for 3 h at 450 OC, results in crystallite growth/ sintering and measurement of a bulk spectrum; see Figure 2a. Sintering is performed in air to prevent loss of oxygen from the crystallite surfaces and formation of a metallic zinc layer.16 We have examined the Burstein shift (see discussion below) as a function of applied potential in a ZnO film prepared from highly confined crystallites at pH 12.0; see Figure 2b. The Burstein shift of a similar film, following firing for 3 h in air at 450 OC, was also examined. Potential-Dependent Spectroscopy of Nanocrystalline ZnO Films. Absorption spectra of a ZnO film (pH 12.0) biased at 0.00, -0.90, and -1.40 V were measured against an air background and are shown in Figure 3a. At sufficiently negative potentials absorbance loss, assigned to a Burstein shift (seediscussionbelow), is observed at wavelengths shorter than 385 nm. The change in absorbance at 360 nm, against a background measured for a sample biased at 0.00 V, was recorded as a function of applied potential at a number of different pHs. The results of these experiments for films at pH 3.0,7.5, and 12.0 are shown in Figure 3b. The onset of absorbance loss is at correspondingly more negative potentials as the pH is increased. The absorption spectrum of a ZnO film (pH 3.0 and 12.0) at 0.00 V was recorded as a background, and difference spectra were subsequently measured as a function of applied potential; see Figure4, a (pH 3.0) and b (pH 12.0). At sufficiently negative potentials the expected absorbance loss at wavelengths shorter than 385 nm is observed. As above, onset of absorption loss is at more negative potentials as pH increases. Similar effects have been observed for transparent nanocrystalline films of Ti02 and assigned to the Burstein shift accompanying accumulation of electrons in the conduction band at sufficiently negative potentials.69* Interestingly for ZnO, unlike TiOz, there appears to be no corresponding absorbance growth in the 385-800-nm region of the spectrum that could be assigned to free conduction band electrons or electrons localized at bulk or surface defects.
Flatband Potential of Nanocrystalline ZnO Films
a
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 11083 ZnO at photon energies greater than E, and no absorption at photon energies below E,. In practice, a(hu) increases exponentially from energies significantly below E,. The observed behavior being well described by Urbach’s rule, eq 2, where c is a constant at a given temperature.21
0.5 pH 12.0
a(hu) = a(E,) exp[$(hu 360
330
Wavelength (nm)
390
b
- E J ] : hv < Eg
(2)
Transitions are accounted for by simultaneous creation of an exciton and absorption of a phonon.22 At low temperature it is possible to resolve a series of peaks in the ZnO absorption edge.23 These absorption features are assigned to discrete states of three Wannier e x c i t 0 n s . 2 ~ A ~ ~general theory of exciton absorption in media of high dielectric constant, where electron-hole interaction is weak, was developed by Elliott based on an effective mass approximation.28The Bohr radius, re, of the first excitonic state ( n = 1) is given by eq 3
rB = eh2/peflre2 (3) The corresponding binding energy EB ( n = 1) is given by eq 4 I
-0.2 -1.5
J
’
’ ’ I.
-1.0
’
’
-0.5
Potential (V, SCE)
0.0
F i p e 3. (a) Absorbance spectra of a transparent nanocrystallineZnO film (pH 12.0) biased at (1) 0.00, (2) -0.90, and (3) -1.40 V (SCE). (b) Absorbance loss monitored at 360 nm as a function of applied potential for nanocrystallineZnO film at pH 3.0, nominally pH 7.6 and pH 12.0.
a
E , = psfi4/8c2h2
(4) where c = ~ ~ (E,€ is0 the dielectric constant at optical frequencies) and pcm is the reduced effective mass of the electron-hole pair given by eq 5 pCff= m*,m*h/(m*, + m*J
(5) m*, and m*h are the effective masses of the electron and hole, respectively. These exciton transitions will be found at energies given by eq 6
E,, = E , - E ,
-0.554 350
I
550
450
Wavelength (nm)
b
e0 a
-0.35 pH 12.0 -0.55
550
450
350
Wavelength (nm) Figure 4. (a) Difference spectraof transparentnanocrystallineZnO film (pH 3.0) biased at (1) 0.00, (2) 4 . 6 0 , (3) -0.80, (4) -1.00, and (5) -1.20 V (SCE). (b) Difference spectra of transparent nanocrystalline ZnO film (pH 12.0) biased at (1) 0.00, (2) -0.80, (3) -1.00, (4) -1.20, and (5) -1.45 V (SCE). Discussion Optical Spectroscopy of ZnO. The band structure in ZnO (hexagonal wurtzite) has been studied extensively.” The lowestenergy absorption of this semiconductor is a direct transition between parabolic bands at E, (3.2 eV at 300 Q.18 The predicted relationship between the absorption coefficient a(hu) and the energy of the exciting photon for a direct semiconductor is given by eq 1.19 a(hv)
Ad-:
hv > E,
An expression for the coefficient A has been derived by Bardeen et al.20 Equation 1 predicts a steeply rising absorption edge for
(6)
The Bohr radius of the lowest-energy exciton in bulk ZnO ( n = 1) is calculated to be 2.5 nm using eq 3 and values form*, = 0.24, m*h = 0.45,and t, = 3.7. The corresponding binding energy is 0.156 eV ( n = 1). Potential-DependentOptical Spectroscopy of Nanocrystalline ZnO Films. The spectral changes accompanying accumulation of electronsin the conduction band of a transparent nanocrystalline Ti02 film have previously been studied.8 Briefly, we observe an underlying absorbanceassigned to free conduction band electrons, a pH-independentmaximum at about 900 nm assigned to electrons trapped at oxygen vacancies, and a pH-dependent maximum (550 nm at pH 3.0 and 850 nm at pH 10.0) assigned to electrons trapped at surface Ti4+sites. At wavelengths shorter than 385 nm we observe an absorbance loss assigned to a Burstein shift, a consequenceof band filling under accumulation conditions.On the basis of the measured absorbance by free conduction band electrons at a given applied potential, it proved possible to determine the number and energy of free charge carriers present in the conduction band of Ti02 and, therefore, also to determine vfb.
To extend this approach to nanocrystalline ZnO films, it is necessary to understand the spectroscopicchanges that accompany accumulation of electrons in the conduction band of this semiconductor. The absorption loss at wavelengthsshorter than 385 nm in nanocrystalline ZnO films under accumulation conditions, shown in Figure 3a, is assigned to a Burstein shift. Initial justifications for this assignment are the similarity of the observed behavior to that reported for Ti02 under potentiostatically controlled accumulation conditions698and for ZnO colloidal dispersionsunder bandgap i l l u m i n a t i ~ n . ~More ~ J ~ importantly, we consider the spectral changes accompanying accumulation of electrons in the conduction band presentedin Figure 3b. Plotting the potential at which measured absorbance loss is 0.05 against pH yields a straight line whose slope is 58 mV/pH unit; see Figure 5. This is as expected for a metal oxide semiconductor electrode in aqueoussolution. The energy of the conductionband
Redmond et al.
11084 The Journal of Physical Chemistry, Vol. 97,No. 42, 1993
a
-0.50
cwr.-0.998
0 v)
104 -0.5
”
0.0
0.5
”
1.0
Ef - Ecb (ev)
’ 1.5
-02.10
4
6
8 1 0 1 2 1 4
PH Figure 6. (a) Logarithm of conduction band electron density, bg(n&), plotted against the differencebetween the Fermi level, Ef,and the energy of the conduction band edge;&, for Ti02 and ZnO (eq 7). (b) Calculated absorption coefficient K for Ti02 and ZnO assuming Ef- &b = 0.5 eV
b
-
2
(es 9). 2e+5
coefficient ( K ) of ZnO and Ti02 at some frequency (w).z5
E le+5
/‘
Oe+O 0
1000
2000
3000
Wavelength (nm) Figure 5. Plot of electrolyte solution pH against potential (SCE) at which measured absorbance loss in Figure 3b, monitored at 360 nm, is 0.05.
at the semiconductor-electrolytesolutioninterface is determined by a proton adsorption-desorption equilibrium establishedunder such conditions.12 This demonstrates a quantitative relationship between the magnitude of the measured absorbance loss at a given applied potential and V, entirely consistent with assignment of the observed absorption loss to a Burstein shift induced by band filling. Interestingly, if the potential at which the measured absorbance loss is 0.2 is plotted against pH, the line connecting points measured at pH 3.0 and pH 12.0 still has a slope of 58 mV/pH unit. However, at pH 7.5, the potential at which the measured absorbanceloss is 0.2 V is more negativethan expected. This effect is illustrated in Figure 3b by indicating the expected relationship (dashed line) between the applied potential and absorbance loss at pH 7.5. In fact, the ZnO film possesses an effective surface pH of 9.2 due to reduction of H+at the surface of the electrode. A similar effect observed for Ti02 has been discussed in detail el~ewhere.~ Unlike TiO2, no absorbance which could be assigned to free conduction band electrons is detected for ZnO films; see Figure 3a.Q Failure to observe such an absorbance can be accounted for. We calculate the number of electrons present in the conduction band, neb, of a transparent polycrystalline Ti02 or ZnO film as a function of the difference between the Fermi level Erin the semiconductor electrodeand the energy of theconduction band edge Ecbr using eq 7.3l
(7) The density of states in the conduction band, gcb(E), is approximated by eq 8
&.,(E) = (21E - Ecb1)”2[8?r(m*,)3/2/h3]
(8) where m*c = 5.6 for TiO2. These calculations, whose results are represented schematically in Figure 6a, predict that the number of electrons present per unit volume in Ti02 for any value of E f more negative than E c b is 113 times greater than present in ZnO. Equation 9, based on classical Drude theory as applied to a semiconductor,permits calculation of a value for the absorption
po is the vacuum permeability, T is the relaxation time of the semiconductor,and p is the electron mobility. Values used were the following: p = 1.9 X 10-3 mz V-l s-1 (ZnO) and 3.0 X 10-5 m2 v-1 s-1 (TiO2); = 6.3 for Ti02 (square of visible refractive index). The value of &b used was that calculated for Ef - E c b equal to 0.50 V using eq 1. The results of these calculations are represented schematicallyin Figure 6b. Since the Ti02 and ZnO films were prepared in a similar manner, from particles having the same average diameter, we would expect absorbance by the Ti02 film to be about 26 times that of the corresponding ZnO film at 750 nm based on the relative magnitude of the calculated values for K. This prediction is in good agreement with our measured spectra. Generally,the electronicand optical properties of transparent ZnO films have been examined in detail.32.33 Absorbance features which might be assigned to electrons trapped at bulk or surface defects appear to be absent. It is known that ZnO possesses lattice defects as a consequence of oxygen vacancies and interstitial Zn at0ms.lJ~9~~ These defects account for ZnO being an n-type semiconductor. (The existence of defects due to zinc vacancies and interstitial oxygen atoms has also been ~uggested.3~)The likelihood of lattice defects is increased by formation of these crystallites at low temperatures. Furthermore, a possible consequence of the synthetic method used is charge compensation by Li+atoms at oxygen vacancy or interstitial zincatomdefects.35 The fact that no absorbancewhich could be assigned to electrons localized at either bulk or surface defects is observed suggests either that the defect concentration is small or that the associated optical cross section is too low. Possible mechanisms of charge conduction within and between the sintered crystallites of ZnO films similar to those used in these studieshave been discussedby other workers.36.37 Generally, however, it is accepted that the defects present in ZnO thin films, and consequentlythe mechanism of charge carrier transport, are strongly dependent on the method of preparation.17 Therefore, work directed toward characterizing the defect and transport properties of the ZnO films used in these studies is in progress. Determination of V, for Nanocrystahe ZnO Films. As a consequenceof there being no measurablefree electron absorbance by ZnO, it has been possible to accurately study the relationship between the Burstein shift and the applied potential at a number of different pHs. Such data may be analyzed to obtain a value for V, at each pH and therefore to obtain a quantitative relationship between the two. The relationshipbetween the magnitude of a dynamic Burstein shift (AE)and the number of electrons present in the available
Flatband Potential of Nanocrystalline ZnO Films
The Journal of Physical Chemistry, Vol. 97,No. 42, 1993 11085
a
states of the conduction band is given by eq hE = (1
+ m*,/m*h)(Ef - E,.. - 4kBT)
pH 12.0 pH 3.0
(10)
where Efisthe Fermi level and Ecbis the energy of the conduction band edge. El - E c b is approximated by eq 11 d 0.00 -0.4
We note that in eq 10 the quantity 4 k ~ accounts T for thermal excitation of electrons accumulated in the conduction band from states whose energies are less than Efto states whose energies lie above Ef. In the experiments for which results are reported, accumulation of electrons in the conduction band is potentiostatically controlled. Consequently, it may be assumed that all states which lie below Efare filled and all those which lie above are vacant. Consequently, AE is given by eq 12
AE = -e,,(l
+ m*e/m*h)(Vf-
vcb)
(13) where eo is 1.602 X J/eV. We assume that the Fermi level in the semiconductor electrode is at the same energy as the Fermi level in the conducting glass substrate. From our experiments we know the potential difference between the Fermi level in the bulk semiconductor and the potential of the bulk electrolyte solution, i.e., (Vf - Vbulk). However, A E will depend on the potential drop across the space charge layer, Le., on (Vf - Vsurface).We can relate the quantity (Vf- Vu,,) to (Vf- Vsurfaa)by taking into account thecapacitance of the space charge and Helmholtz layers, that is, Csc and CH, respectively. If these two capacitances are assumed to be in series, than eq 13 relates (Vf- ‘Vbulk) to (Vf - Vsurfacc).
The correction to the measured quantity (vf - VbuIk), given by eq 14, is calculated individually for each potential, CSCbeing a function of (Vf- Vsurface).The relationship between CCSand (fi - Vsurfaa)has been discussed in detail elsewhere.8 CCSwas found to be between 0.006 and 0.007 F m-2 at all applied potentials for which data are reported. The value for CHused was 0.85 F m-2.39 It is assumed that CH is independent of (Vf - Vsurfaa)and that the potential drop through the electrolyte solution can be neglected as a consequence of the cell geometry. The intrinsic carrier concentration was taken as 1024 m-3. This represents the lower end of the range of reported values for polycrystalline ZnO films ( 1023-1 026 m-3),3*,33 The lower end of the available range is chosen to account for the known effect of Li+ atoms, expected to be present in the ZnO ~rystallites.1~The Burstein shift can be estimated from the bleach width at half-amplitude of the data shown in Figure 4. Plotting the Burstein shift (AEleo) against (Vf- Vsurfaa) results in a straight line relationship; see Figure 7a. The intercept of such a plot with the axis represents the value of (Vf- VSVsurfacc) for which the Burstein shift is zero, i.e, Vh. If such an analysis is performed for data measured at pH 3.0 and pH 12.0, then it is possible to prepare a plot such as that shown in Figure 7b, from which we obtain the following expression yielding an absolute value for Vh as a function of pH: Vh = -0.24 - 0.057pH (V,SCE)
(15)
The expression given above yields a value for Vha t pH 13.3 equal to 1.00 V, in good agreement with that measured for singlecrystal zinc oxide by Lohmann using the MottSchottky method, 1.09 V.” The results obtained in this paper were discussed in terms of amphoteric dissociation of ZnOH surface groups. We
SCE)
- 1.6
-
+
hE = (1 m*e/M*h)(Ef - E&) (12) We can write eq 12, as follows, in terms of electrochemical potentials to yield eq 13.
IVf - Vsu;:f(V,
1
Wb -.0.24.0.057xpH (V, SCE)
0.0
0
2
4
6
8 1 0 1 2 1 4
PH Figure 7. (a) Burstein shift AElt-0, as determined form data in Figure 4, plotted against (Vf-V8dw). (b) Flatband potential Vfidetermined from (a) plotted against pH of electrolyte solution. note that use of the MottSchottky method to determine Vb for polycrystalline ZnO films is likely to present significant difficulties. 12 Burstein Shift in Confined Nanocrystalline ZnO Films. The value of 2.5 nm for rB is indicative of the diameter of a crystallite that would be expected to show quantum confinement effects. Crystallitesused to form films for which experiments are reported had an average diameter of 13 nm, consistent with measurement of a bulk spectrum prior to and following sintering at 450 OC. Where a film was formed using freshly prepared crystallites (see Figure 2a), the observed onset for absorbance is significantly blue-shifted. Furthermore,the Burstein shift observed for a given applied potential is also significantly blue-shifted over that observed at the same potential in a bulk film. Such observations are forming the basis of a study currently in progress to examine whether the blue shift which accompanies increased free electron concentration in a confined particle is different in nature to that observed in the film constituted from bulk particles.
Conclusions The spectroscopy of transparent polycrystalline ZnO films has been studied between 300 and 800 nm under accumulation conditions. Observed spectroscopic changes can be accounted for in terms of a blue shift produced by band filling, i.e. a Burstein shift. In agreement with the calculated spectrum, no absorbance which could be assigned to free conduction band electron was detected. Furthermore, no absorbance which could be assigned to electrons localized at defects was observed. The absence of any detectable increase in absorbance upon accumulation of electrons in the conduction permitted careful examination of the associated Burstein shift in bulk ZnO films. On the basis of the relationship between the measured Burstein shift and the applied potential, it was possible to determine the flatband potential for a polycrystalline ZnO film. This approach offers the prospect for determination of the flatband potential of a wide range of polycrystalline semiconductor films. Further, few assumptions are necessary to extract the required parameter. The apparent blue shift of the Burstein shift in unfired films prepared from confined ZnO particles is particularly interesting. Also of interest is the absence of spectral changes in the visible upon accumulation of electrons in the conduction band. This offers the prospect of simultaneously monitoring, without mutual interference, spectroscopic changes assigned to accumulation of electrons at the
11086 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
electrode-electrolyte solution interface and reduction of a redox species,by the accumulatedelectrons, at the same interface. Work is currently in progress in each of the above areas.
Acknowledgment. We thank Hewlett Packard Ireland and
EOLAS (The Irish National Board for Science and Technology) for their support of this research.
References and Notes (1) Roth, A. P.; Webb, J. B.; Williams, D. F. Solid Stare Commun. 1981,39, 1269. (2) Liu, C.; Bard, A. J. Phys. Chem. 1989, 93, 7749. (3) ORegan, B.; Graetzel, M. Nature 1991, 353, 737. (4) Hotchandani, S.; Kamat, P. Chem. Phys. Lett. 1992, 191, 320. (5) ORegan, B.; Moser, J.; Anderson, M.; Graetzel, M. J . Phys. Chem. 1990, 94, 8720. (6) ORegan, B.; Graetzel, M.; Fitzmaurice, D. Chem. Phys. Lett. 1991, 183, 89. (7) O’Regan, B.; Graetzel, M.; Fitzmaurice, D. J. Phys. Chem. 1991,95, 10525. ( 8 ) Rothenberger, G.; Fitzmaurice, D.; Graetzel, M. J. Phys. Chem. 1992, 96, 5983. (9) Kavan, L.; Stoto, T.; Graetzel, M.; Fitzmaurice, D.; Shklover, V. J.
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