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J. Phys. Chem. C 2007, 111, 13092-13102
Surface-State-Mediated Electron Transfer at Nanostructured ZnO Multipod/Electrolyte Interfaces Meera Parthasarathy, Niranjan S. Ramgir,† Bhaskar R. Sathe, Imtiaz S. Mulla, and Vijayamohanan K. Pillai* Physical and Materials Chemistry DiVision, National Chemical Laboratory, Pune, Maharashtra, India 411 008 ReceiVed: April 5, 2007; In Final Form: June 23, 2007
Redox kinetics of cyanoferrate(III) species adsorbed at an n-type ZnO multipod/electrolyte interface is explored using electrochemical techniques like cyclic voltammetry and impedance spectroscopy. The electrochemical impedance results are analyzed using a fluctuating energy level model, assuming isoenergetic tunneling of majority carriers through the Helmholtz layer. A shift in the slope of Mott-Schottky plots (Csc-2 versus E) together with evidence from cyclic voltammetry shows that the electron-transfer process is mediated by surface states formed because of the adsorption of ferricyanide ions (as evident from the results of Fourier transform infrared spectroscopy). More significantly, the pH of zero charge (point of zero zeta potential, pzzp) of ZnO multipods is found to be 4.5 (from capacitance vs pH plots) compared to that of bulk ZnO (pH 9.5), which could be explained on the basis of a lowering in the work function of the nanostructured semiconductor and its consequent susceptibility to the formation of surface states. This is in excellent agreement with our earlier observation of ultralow threshold field emission with this material in the light of the linear dependence of pzzp with work function of the electrode material. The flat-band potential of the nanostructures is found to be 200 mV more negative than that reported for bulk n-type ZnO electrodes, indicating a higher doping density in the former. A three-dimensional mapping of charge distribution in the surface states is attempted by correlating the capacitance response of the system subjected to a sinusoidal potential modulation to the semiconductor electrode with that resulting from a systematic variation of the redox potential of the dissolved acceptor (achieved by varying the pH of the electrolyte) which further reveals the polyenergetic nature of the surface states.
1. Introduction In recent years, studies on the reactivity of wide band gap oxide semiconductors like ZnO and TiO2, especially in nanostructured form for applications in photosensitized solar cells,1 photocatalytic degradation of atmospheric and water pollutants,2-4 field emission devices,5 solid-state gas sensing,6 and so forth, have become an area of immense research interest owing to the excellent chemical and mechanical stability of the materials and flexibility of processing in the form of large area thin films and low cost fabrication.7,8 The reactivity of the semiconductor catalysts could be tremendously improved by increasing the surface-to-volume ratio of the material as a function of size and shape. For example, in photocatalytic reactions, nanostructured ZnO catalysts with confined geometries like tetrapods9 (30 nm mean diameter) and needles10 (60 nm mean diameter; 1.1 µm length) are found to have improved kinetics compared to that on bulk materials because of the lesser chance of the recombination of photogenerated electron hole pairs. Particularly, Kamat and co-workers have established at many instances the efficiency of ZnO nanostructures as photocatalysts for environmental remediation.11-13 More interestingly, the spatially restricted geometry and morphology of these nanostructures are also found to have significant effect on their field emission characteristics. For instance, in our earlier studies we have * To whom correspondence should be addressed. Phone: 91-02025902270. Fax: 91-020-25902636. E-mail:
[email protected]. † Current address: Department of Electrical Engineering, University of South Florida, Tampa, FL.
observed ultralow threshold field emission behavior from anisotropic ZnO multipods with 60 µm long arms with a tip apex of 24 nm.5 On the other hand, the areal density of ZnO nanowires is also found to affect the field emission characteristics like the turn-on voltage and emission current density.14 However, reproducibility of the electronic properties of a given nanostructure requires a thorough understanding of its purity and the effect of adsorbed species on the surface structure and energy distribution. Often, localized charges (called surface states) are created within the band gap region either because of surface inhomogeneities like nonstoichiometry or because of the selective adsorption of foreign species in addition to the abrupt termination of lattice periodicity. The adsorbed species could be either a sensitizer attached intentionally to improve the performance of the photocatalyst or an inadvertent impurity arising because of the process of fabrication. Such surface states are found to strongly influence the electronic properties as well as the redox kinetics at the semiconductor surfaces and interfaces, the potential implications of the effects being especially noticeable in the case of nanostructured materials.15 A typical example for the former case is a report by Zhang et al. wherein they have shown how to control conductivity by effectively tuning the surface states on a silicon membrane of a few nanometer thickness for applications in micro- and nanoelectromechanical systems.16 The importance of surface states in semiconductor device applications is also explicit from a recent report by Boland.17 On the other hand, the effect of surface states on redox kinetics at semiconductor nanocrystals
10.1021/jp072695x CCC: $37.00 © 2007 American Chemical Society Published on Web 08/10/2007
Electron Transfer at Nanostructured ZnO Multipods could be understood from a report by Wan et al. wherein they have studied the influence of intrinsic surface states on electron/ hole recombination rate in ZnO nanotetrapods using photoluminescence and found that the current decay is very slow compared to that in bulk ZnO because of a surface-state-related recombination process, thereby improving the photocatalytic effect of the nanomaterials.9 Similarly, ruthenium ions chemisorbed on the surface of n-GaAs are found to reduce the surface recombination velocity of charge carriers by 1 order of magnitude as reported by Nelson et al.18 In addition, the adsorption of foreign species also changes the surface properties so drastically that even the work function of metal crystals in high vacuum is found to shift in the extreme case.19 For example, Schwettman et al. have observed an enhancement in x-radiation count rate by a factor of 104-106 and the corresponding dc electron field emission characteristics in superconducting cavities of Bi, Nb, Cu, and W because of resonant tunneling through adsorbate surface states.20 Another interesting example is a report by Biancarde et al. wherein they have achieved longlived photoinduced charge separation by the coadsorption of sensitizers and electron acceptors on TiO2 nanoparticles.21 Thus, all the above cases exemplify the necessity to explore the effect of surface states (i.e., localized surface energy levels formed by the adsorbed species) on the redox kinetics at a nanostructured semiconductor-electrolyte interface, which would be useful for optimizing their performance in aforementioned applications. Various methods based on photoluminescence have been adopted for analyzing surface states on semiconducting materials including surface photovoltage measurements (SPV) and photoinduced and microwave conductivity (PMC). For example, a report by Alperson et al. on the mapping of intrinsic surface states in CdSe quantum dots (QDs) by conductive scanning force microscopy demonstrates an elegant way of characterizing the position of surface states in the band gap of the material.22 However, the above technique provides information only about the position and distribution of surface states and not on their reactivity. In this connection, electrochemical impedance technique is identified as a more suitable and subtle tool to explore the chemical reactivity and relaxation dynamics of the surface states and their coupling to the interfacial electron-transfer process.23 Although the electrochemical experiments are often carried out under illumination, a quantitative analysis of the photocurrent behavior unfortunately provides only limited information on the kinetics of redox process because the photocurrents are determined by light excitation.24 Conversely, studying the redox kinetics involving majority carriers in the dark provides more reliable information on redox kinetics and surface-state relaxation processes. However, the latter results would be also useful to understand the photoprocesses, as it is possible to compare quantitatively the majority carrier processes at n-type electrodes to minority carrier processes at p-type electrodes of the same material or vice versa using a quasiFermi level approach developed by Memming and co-workers.25,26 As far as the electrode materials are concerned, ZnO is one of the well-characterized semiconductor electrodes, because many redox reactions exhibit ideal current-potential behavior at ZnO-electrolyte interfaces.27-29 Although a considerable number of reports exist on the redox behavior of single crystalline (bulk) ZnO electrodes,27-30 a fundamental understanding of electron-transfer (ET) processes at nanostructured ZnO electrode-electrolyte interfaces, where surface states are expected to be more dominant, is still at its infancy perhaps
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13093 because of the complications in the systematic interpretation of their electrochemical data. In addition, the geometric restrictions easily attainable in ZnO nanostructures owing to the anisotropy inherent in its Wurtzite structure could have interesting implications on its electrochemical behavior.31 Consequently, we report here for the first time some remarkable results of our electrochemical investigations of electron-transfer kinetics at n-type ZnO multipod/electrolyte interfaces (which showed ultralow threshold field emission in our previous studies5) using cyclic voltammetry and electrochemical impedance techniques. Interfacial electron transfer at cyanoferrate(III) ions adsorbed on ZnO multipods is chosen as a model system primarily because of the availability of data for this well-characterized bulk single crystalline ZnO28,29 to validate the approach used in the present investigation. However, the protocol could be extended for other sensitizers like organic dye materials adsorbed on ZnO nanocrystals, which would be useful to understand interfacial charge-transfer dynamics in photosensitized ZnO solar cells, facilitating significant improvements in photoconversion efficiencies. Furthermore, we present here a unique correlation between the semiconductor/vacuum interface (which decides the field emission characteristics) and the semiconductor/electrolyte interface (important in electrochemical devices) since this, in fact, is one of the main purposes of choosing ZnO multipods with well-demonstrated enhanced field emission characteristics. Such a correlation which bridges the concepts of semiconductor physics and electroanalytical chemistry would be of unprecedented importance to develop smart semiconductor nanostructures with predetermined efficiencies. 2. Theoretical Background Although the thermodynamic treatment of galvanic cells is similar for both the metal and semiconductor electrode/ electrolyte interfaces, the basic concepts governing chargetransfer kinetics at the electrode surface vary considerably with the band structure of the electrode material. Because of much lower concentration of charge carriers (1013-1017 cm-3) in semiconductors, any excess charge generated because of an externally applied voltage would extend quite far into the interior (∼100 angstrom) over the so-called “space-charge layer” and the electrical field strength drops to much lower values (∼104 V cm-1) than in a metal-electrolyte interface (∼107 V cm-1). Hence, the applied voltage at a semiconductor electrode controls primarily the probability factor (entropy factor) of a surface reaction and not the energy factor as in the case of metal electrodes.32 Also, the great difference in binding energy between electrons in conduction band and the carriers in the valence band (called “holes”) results in variation in electrontransfer pathways under illumination from those in the dark, a concept which has interesting consequences in photoelectrochemistry. According to the theories of electron-transfer kinetics, the electron-transfer process is considered to have the maximum probability when the energy levels of the initial and final states of the system coincide (resulting in a Frank-Condon type transition).33 This essentially requires a perfect overlap between the quantum states in the semiconductor energy bands and the donor/acceptor levels of redox species dissolved in the electrolyte. Such an overlap is most often unfavorable in which case shallow surface states located within the band gap become necessary for charge transfer. In an ideal case, when the surface states are absent, only electrons that are thermally excited to the conduction band for an n-type material could get involved in a majority carrier process, and use of a wide band gap (>3
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eV) semiconductor indeed ensures this. Thus, the net flux of electrons from the conduction band to acceptor species dissolved in the solution (under forward bias) is given by34
J(E) ) -qket[A]ns
(1)
where J is the current density (A cm-2), E is the applied potential (V) versus saturated calomel electrode (SCE), q is the elementary charge (1.602 × 10-19 C), ket is the electron-transfer rate constant (cm4 s-1), [A] is the acceptor concentration (cm-3), and ns is the electron concentration at the semiconductor surface (cm-3) implying a second-order rate expression (which in turn denotes first order in the acceptor concentration as well as in the electron concentration at the surface). The variation of electron concentration at a doped semiconductor surface with the applied potential follows the Boltzmann statistics as given below.
ns ) Nd*exp(-e*Vb/kBT)
(2)
where kB is Boltzmann’s constant, T is the temperature, Nd is the dopant density, and Vb is the height of the potential barrier in the space-charge layer which may be expressed in terms of the semiconductor’s Fermi level and the redox potential of the solution. On the other hand, when adsorption of ionic species on the semiconductor surface leads to the formation of surface states (surface states are electronic energy levels located at the surface capable of transferring charges with the electrode and the solution), localization of electrons within such states would affect the distribution of interfacial potential difference, ∆φ, over the depletion and Helmholtz layers.35,36 As a result, the distribution of a small applied potential modulation, δ∆φ, over these two layers becomes time-dependent and more significantly coupled to the interfacial kinetics, leading to an additional resistance-capacitance (RC) component in the impedance spectrum with a different time constant. Assuming that majority carriers tunnel through the Helmholtz layer isoenergetically (according to the Marcus-Gerischer model of semiconductor/ electrolyte interfaces)37 and that the electronic states of the redox system follow the fluctuating energy level model,38 electrical equivalent circuits for direct and surface-state-mediated electron transfer at a semiconductor/electrolyte interface differ as shown in Figure 1a and 1b.38 The implications of the equivalent circuit developed for surface-state-mediated electron transfer are (a) the first component corresponds to the double layer capacitance. Since the transfer function, ∆φH(ω)/∆φ(ω), is frequency dependent, it is not possible to split the contributions of depletion layer from the Helmholtz layer. (b) The second component denoted by R in the circuit diagram is purely resistive. (c) The third component consisting of a series RC component (Rss, Css) in parallel to a series RL component (RL, L) corresponds to the relaxation in the occupancy of surface states which often leads to a capacitive loop according to the basic assumptions of the treatment.38 The coil element, L, indicates the involvement of inductive elements corresponding to electron-transfer loops associated with surface states. Thus, the semicircle observed in the complex plane impedance plots is related to the interfacial redox kinetics through the relaxation of surface-state occupancy. 3. Materials and Methods ZnO multipods were prepared by a vapor deposition method as reported earlier.5 All reagents were of analytical grade. Potassium ferricyanide and potassium ferrocyanide were pro-
Figure 1. (a) Equivalent circuit for direct electron transfer from the semiconductor conduction band to the acceptor in solution corresponding to a frequency-independent transfer function, ∆φH(ω)/∆φ(ω) ) 1 - CH/(CH + CSC), where CH is the Helmholtz capacitance and CSC is the space-charge capacitance. (b) Equivalent circuit for surface-statemediated electron transfer from the semiconductor conduction band to the acceptor in solution corresponding to a frequency-dependent transfer function, ∆φH(ω)/∆φ(ω) ) [1 - CH/(CH + CSC)] + γs(kBT/e)[-θ(ω)/ ∆φ(ω)], where γs describes the influence of the occupancy of surface states on the distribution of interfacial potential drop over the depletionand Helmholtz-layers and θ(ω) corresponds to the harmonically modulated occupancy of surface states. The charge-transfer resistance Rct is given by (R-1 + RL-1)-1. (c) Circuit model assuming CH . Csc and by combining R and RL into a single parallel resistance.
cured from Aldrich chemicals while potassium chloride and sodium hydroxide were obtained from Qualigens fine chemicals, Mumbai, India. Concentrated HCl was obtained from Ranbaxy Fine Chemicals pvt. Ltd., New Delhi, India. Deionized water (Millipore 18 MΩ) was used for making all the solutions. For electrochemical investigations, a 1.5 mm Pt disk electrode was used as the working electrode, Pt foil was used as counter electrode, and Ag/AgCl was used as reference electrode. The pH of the electrolyte (0.1 M KCl) was adjusted by adding dilute HCl with simultaneous monitoring using a pH electrode purchased from Eutech instruments Pvt. Ltd. The ZnO multipods were dispersed in aqueous Teflon slurry (5 wt %) for better binding and were drop-coated on the Pt electrode, which was used as working electrode for the electrochemical studies. Cyclic voltammetry was performed in dark using a Solartron SI 1287 electrochemical interface operated through Corrware software, and electrochemical impedance was performed (in dark) using a Solartron 1255B instrument equipped with frequency response analyzer, operated through a Zplot software. 4. Results and Discussion 4.1. Cyclic Voltammetry. Figure 3 shows cyclic voltammograms of n-type ZnO multipods (scanning electron microscopy (SEM) image is shown in Figure 2) in the presence of 5 mM K3[Fe(CN)6] in 0.1 M KCl at pH 4 along with that in the absence of redox species in solution. The [Fe(CN)6]4-/[Fe(CN)6]3system is chosen for the analysis to ensure that only the conduction band electrons are involved in the interfacial electron-transfer process. This is possible because the unoccupied energy levels of the above redox species are close to the conduction band edge of ZnO (ECB/q ) -0.34 V vs
Electron Transfer at Nanostructured ZnO Multipods
Figure 2. Scanning electron micrograph of ZnO multipods synthesized by vapor deposition method (recorded separately, not on the electrode surface), which was found to have ultralow threshold field emission behavior in our earlier studies.
Figure 3. Cyclic voltammograms of ZnO multipods in the presence of 5 mM K3[Fe(CN)6] in 0.1 M KCl maintained at pH 4 and their variation with potential cycling at a scan rate of 25 mV/s with Ag/ AgCl, KCl sat. reference electrode, and a Pt flag counter electrode; the black line indicates the CV of ZnO multipods in 0.1 M KCl in the absence of K3[Fe(CN)6].
SCE).30,28 The compact nature of the ZnO film is ascertained by the absence of redox peaks at an E1/2 of 0.3 V (vs Ag/AgCl, sat. KCl ref) corresponding to that of a bare Pt electrode (Figure 1, Supporting Information). Further, the cyclic voltammograms are found to exhibit interesting variations with initial potential cycling, which then becomes steady with an E1/2 of 0.74 V. Further, on the basis of earlier reports on the irreversible adsorption of ferricyanide ions on ZnO single-crystal electrodes,28,29 the initial cycle dependence could be explained as follows. When the ZnO surface is fresh, it is very active toward adsorption and subsequent reduction of ferricyanide. On successive cycles, both the anodic and cathodic peak currents (Ipa and Ipc, respectively) decrease, accompanied by a concomitant positive shift in the Epc values. While the decrease in peak current could be ascribed to the partial blocking of ZnO surface by adsorbed species, the positive shift in the cathodic peak potential indicates that the reduction of ferricyanide species becomes thermodynamically more favorable signifying perhaps the formation of some intermediate energy levels on the semiconductor surface, which could mediate the reduction of the adsorbed species. Also, the irreversible nature of the redox
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13095
Figure 4. Variation of cathodic and anodic peak current density with pH of the electrolyte as derived from the respective cyclic voltammograms at 25 mV/s of ZnO multipods attached to Pt electrode with Ag/ AgCl, KCl sat. reference electrode, and a Pt flag counter electrode.
peaks with a peak potential separation of 200 mV is as expected for a redox process at an n-type semiconductor/electrolyte interface.29 Further, to understand the individual interactions of the oxidized and reduced forms of the electroactive species with the ZnO surface, the voltammetric behavior is studied by varying the pH of the electrolyte, because ZnO is known to alter its surface charges depending on the pH of the medium. For example, at pH values below its isoelectric point, ZnO is known to get protonated and to develop a positive surface charge.39 Such an analysis shows that the cathodic peak current density (Ip,c) (measured after the voltammograms had become steady, independent of cycles) is almost pH independent whereas the anodic counterpart increases exponentially with increasing pH as shown in Figure 4. The pH independence of the cathodic current signifies the adsorption of ferricyanide ions on the surface of ZnO multipods prior to electron transfer from the conduction band to the species, thereby making the process insensitive to changes in mass transfer accompanying the pH change. This observation is also in excellent agreement with earlier reports on irreversible adsorption of ferricyanide ions on ZnO single-crystal electrodes.28,29 Conversely, the pH dependence of anodic peak current implies that the reduced form, that is, ferrocyanide species, does not adsorb specifically on the semiconductor surface. On the other hand, it becomes susceptible to changes in the mass transport kinetics, arising because of variations in surface charge density on ZnO with electrolyte pH and subsequent alterations in the concentration profiles of the electroactive species in the diffusion layer. At the same time, the effect of the anisotropic electrode shape on the mass transport behavior of the electroactive species from the bulk of the electrolyte to the electrode surface could not be neglected altogether in the nanostructured electrode. For instance, although the shape of the voltammograms in the first few cycles (Figure 3) indicates a diffusion-controlled regime (i.e., the diffusion layer thickness is considerably smaller than the electrode dimensions), wherein the current density is least affected by the electrode shape, the current density for the anodic processes approaches a steady-state (it could be a quasi-steady state also) condition in the fifth cycle, that is, at a longer time scale. This especially signifies the effect of the nanosized multipod “tips” on the diffusion fields of the electroactive species, which results in a steady-state/quasi-steady-state condition at a longer time scale. Further, the sensitivity of current-
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Figure 5. Variation of the open circuit voltage (OCV) of ZnO multipod/electrolyte interfaces with pH of the electrolyte medium at 25 °C. A maximum could be observed at pH 4.2 corresponding to the pzzp of the ZnO multipods.
potential behavior of the anodic process alone to the nanosized tips is in accordance with the interpretation of pH dependence of current densities discussed above that only the reduced form (i.e., ferrocyanide) of the electroactive species is sensitive to mass transport effects as it is not irreversibly adsorbed on the electrode surface. Further, the second-order rate constants for the cathodic process calculated for the ZnO multipods are found to be 1 order lower (∼10-18 cm4 s-1) than that reported for ferricyanide reduction at single-crystal bulk ZnO electrodes (1 × 10-17 cm4 s-1).27,41 However, the difference in electrontransfer rates at the ZnO multipods with respect to bulk ZnO could be accounted for by considering the effect of the anisotropic structure of the former on the adsorption of the electroactive species as well as on the subsequent interfacial electron transfer. For instance, the inherent anisotropy in the ZnO multipods may provide discernible sites for ferricyanide adsorption resulting in differences in reactivity. Also, there could be a distribution of surface states depending on the anisotropic morphology. In addition, the thermodynamic potential (open circuit voltage or OCV) of the system shows an interesting variation with the electrolyte pH as shown in Figure 5. As evident from the OCVpH plots, the thermodynamic feasibility of the redox process is found to be maximum at pH 4.2, the significance of which could be appreciated by a comparison with the impedance results (particularly the space-charge capacitance versus pH shown in Figure 9 and charge-transfer resistance versus pH in Figure 10) discussed in the next section. Moreover, the ZnO nanostructures are found to be considerably stable in the pH range employed here within the time scale of the experiments (Figure 4, Supporting Information). In addition, the potential window is also chosen on the basis of the reported chemical stability of ZnO in this pH range (pH 4-10).41 Having hinted on the adsorption of ferricyanide on ZnO, we proceed further to characterize its effect on the band structure of ZnO multipods in the following section using electrochemical impedance technique. 4.2. Electrochemical Impedance Analysis. Figure 6a shows the Nyquist plots (-Z(Im) vs Z(Re)) of ZnO multipods in 0.1 M KCl at pH 4, with and without [Fe(CN)6]4-/[Fe(CN)6]3couple. In the presence of the redox species, the complex plane impedance plot consists of a curved region in the high-frequency part related to the interfacial redox process, followed by a linear region in the low-frequency part corresponding to the mass
Figure 6. (a) Nyquist plot (-Z(Im) vs Z(Re)) of ZnO multipods in 0.1 M KCl at pH 4 under open circuit condition, with an equimolar mixture of K4[Fe(CN)6] and K3[Fe(CN)6] (5 mM each) (shown in the figure by open circles) and that of ZnO multipods in the absence of redox species (diamond symbol) using a 10 mV rms ac signal with frequency ranging from 10 mHz to 100 kHz. (b) Variation of impedance modulus (|Z|) (in logarithmic scale) versus frequency (in logarithmic scale); the experimental results are shown as points and the line is the fitted plot using the equivalent circuit shown in Figure 1b in the frequency range 150 Hz to 5 kHz. (c) Phase angle versus frequency (logarithmic scale) showing the experimental (points) and the fit result (line) in the frequency range 150 Hz to 5 kHz using the above equivalent circuit (Figure 1b) meant for surface-state-mediated electron transfer at a semiconductor/electrolyte interface.
transport of the species from the bulk of the electrolyte to the electrode surface. Conversely, the complex plane impedance plot of the ZnO multipods in 0.1 M KCl in the absence of the redox species does not show any curved portion in the highfrequency part. Further to understand the contributions to the impedance response from the Pt electrode used as the substrate, the experiments were repeated under identical conditions with a bare Pt electrode, the results of which are summarized in the Supporting Information (section 1). The impedance behavior of the Pt electrode is found to be totally different from that of the ZnO multipods and yields the best fit only with a simple Randles equivalent circuit (consisting of a resistor in series with a parallel RC component) characteristic of metal/electrolyte interfaces. Also, the impedance plots of the Pt electrode had no correlation with the complicated circuit (Figure 1b) used here for analyzing the ZnO multipod/electrolyte interface. Thus, after understanding the nature of the substrate Pt electrode/electrolyte interface, the curved portion in the highfrequency part of Figure 6a is fitted in the frequency range 150 Hz to 5 kHz, using CNLS (complex nonlinear least-square fitting) method to the equivalent circuit shown in Figure 1b meant for surface-state-mediated electron transfer which gave the best fit to the experimental data, among few other possible equivalent circuits.42 The fitting results are shown in Figure 6b
Electron Transfer at Nanostructured ZnO Multipods and Figure 6c in the form of Bode plots, as |Z| and phase angle versus frequency, respectively. More interesting details about the relative contributions from various processes occurring simultaneously at the semiconductor/electrolyte interface is obtained by analyzing the sensitivity of the fits to individual parameters shown briefly in the form of phase angle versus frequency plots in the Supporting Information (section 4). The fitting is found to be more sensitive to variations in surfacestate capacitance (Css) than space-charge capacitance (Csc) as shown in section 4.2 of the Supporting Information except for the maximum deviation noticed in Figure 1c, when Csc is of the same order as that of Css. In addition, the nature of deviation of the fits from the experimental phase angle versus frequency plots also provides considerable insight into the importance of the processes. For instance, variation in RL particularly leads to deviations in the fwhm (full width at half-maximum), which indicates that the RL component corresponds to the relaxation of surface states as proposed in section 2. Particularly, the deviations in the fit results with respect to variations in the RL and Css components clearly demonstrate the involvement of surface states and their importance in the electron-transfer process at the ZnO multipod/electrolyte interface. However, the model in Figure 1b may impose limitations on the fitting procedures because of the presence of several circuit elements, which might in turn lead to overinterpretation. To verify this, a simpler circuit shown in Figure 2 is developed basically from that in Figure 1b using the following fairly reasonable assumptions: (1) CH >> Csc, so that the two capacitors in series could be replaced with a single capacitor, Csc, as the total capacitance is related to the semiconductor only. (2) As the inductance value is not so high in the applied frequency range, R and RL could be coupled to give a net parallel resistance, Rct ) 1/[(R + RL)-1]. Interestingly, the sensitivity of fit to the Csc values analyzed using the revised model (section 5, Supporting Information) indicates the involvement of space-charge layer in the interfacial processes (although the fitting results are similar for both the models). The inadequacy of the earlier model in analyzing this aspect could be mainly due to the difficulty in fixing the Helmholtz capacitance to a reasonable value during the fitting process, as the surface enhancement effects of the ZnO multipods are difficult to be quantified. Hence, in the former model (Figure 1b), the changes in Csc are easily accommodated by changes in CH left as a free parameter while fitting. The purpose of this section is to understand whether the adsorption of the redox species on ZnO leads to shallow energy levels within the band gap of the semiconductor surface leading to the formation of “surface states”. Strictly speaking, the possible discrimination of the space-charge effects from the surface-state contribution using electrochemical impedance analysis has often been a conundrum particularly at a polycrystalline semiconductor/electrolyte interface owing to intrinsic limitations of the model-dependent impedance analysis arising because of contributions from surface inhomogeneities and grain boundaries. Despite such limitations in theoretical analysis, electrochemistry at polycrystalline electrodes continues to be an area of intense investigation as polycrystalline materials of large surface area are far less expensive to manufacture than the single crystals and also because a majority of practical applications ensue the use of these polycrystalline materials.43,44 For example, Vlachopoulos et al. have recently achieved a major leap in this area by preparing polycrystalline TiO2 films with surface roughness factors of several hundreds.45 Similarly, Chaparro et al. have observed better surface reactivity at
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13097 polycrystalline ZnO electrodes compared to that at single crystalline electrodes.46 To circumvent the difficulty in studying polycrystalline ZnO electrodes, the task is performed by coupling two different approaches to delineate the contribution of surface states from that of the space-charge effects in the present work. Although both approaches ultimately lead to a change in the surface charge density of the semiconductor, one involves varying the magnitude of a dc voltage, superimposed with a small amplitude (10 mV rms) ac signal for studying the frequency response of the interface, whereas the other involves changing the pH of the electrolyte solution. The strategy of this coupled approach lies in the fact that in the absence of surface states, both methodologies should result in a similar variation of the differential capacitance of the interface. On the other hand, this may not be the case in the presence of surface states, because the occupancy of surface states is expected to be more sensitive to redox potential changes in the solution side rather than to changes in potential applied to the electrode (since most of the potential drop occurs within the space-charge region), and it must be possible to separate surface-state contribution from space-charge effects by coupling these two results. This could be compared with a similar strategy by Hamers et al. of generating new surface states close to the energy gap by tailoring the HOMO-LUMO gap of molecular species attached to the surface.47 4.2.1. Potential-Dependent Frequency Response and MottSchottky Analysis. The equilibrium condition of a semiconductor/electrolyte interface requires that the Fermi levels of the semiconductor, EF, and the redox species in solution, Eredox, be equal. Hence, as soon as a semiconductor is dipped in an electrolyte, electrons cross the semiconductor/electrolyte interface to achieve equilibrium, resulting in band bending and concomitant changes in the charge distribution at the interface region. The equilibrium state of a semiconductor/electrolyte interface is characterized by an important parameter called the flat-band potential, Efb, which is related to the extent of band bending with respect to the redox potential of species in the electrolyte and hence the potential drop at the space-charge layer at equilibrium. The flat-band potential is most commonly determined by measuring the space-charge capacitance as a function of an applied dc bias, superimposed on a small amplitude ac signal. The space-charge capacitance as a function of electrode potential then follows the Mott-Schottky equation33
Csc-2 ) [2/(qoNDAs2)][E - Efb - (kBT/q)]
(3)
where is the static dielectric constant of ZnO (8.65),30 o is the permittivity of free space, ND is the dopant density of the semiconductor (cm-3), As is the area of the semiconductor electrode, and Efb is the flat-band potential of the semiconductor/ electrolyte interface. Thus, a plot of 1/Csc2 against the applied potential, V, results in a straight line; the potential at which the line intersects the potential axis yields the flat-band potential and the slope yields the dopant density of the semiconductor. However, the flat-band potential determined by the MottSchottky equation may be affected by various factors like high doping concentrations (maximum potential drop occurs in the Helmholtz layer rather than in the space-charge layer), the presence of high density of surface states, and the influence of diffusion-controlled processes.33,48 In this work, we have used the Mott-Schottky equation to analyze the surface states by ruling out the other two factors as given below.
13098 J. Phys. Chem. C, Vol. 111, No. 35, 2007 (1) The contribution from diffusion-controlled processes is made negligible by a proper choice of the frequency range for deriving the capacitance values by fitting with the equivalent circuit. (2) The donor density of ZnO nanowires grown by vaporphase techniques is known to be low (1017-1018 cm-3)49,50 so that the condition CH . Csc is valid (CH: Helmholtz capacitance; Csc: space-charge capacitance) to allow the maximum potential drop to occur within the space-charge layer. Additional constraints are imposed on the applicability of the ideal Mott-Schottky equation when the semiconductor electrode is in nanometer dimensions. For example, Mora-Sero et al. have found that the ideal Mott-Schottky relation will not hold for ZnO nanowires and have modified the Mott-Schottky relation to account for the circular depletion layer that will grow from the surface toward the center of the wire at changing bias potential.51 Although the experimental conditions (substrate electrode: fluorine-doped tin oxide) of their study differ widely from our conditions, the theoretical treatment has important implications on semiconductor electrochemistry using nanostructured electrodes. However, the dimensions of the ZnO multipods used in the present investigations (diameter of the arms: 2-3 µm; length of the arms: 10 µm) are considerably greater than the depletion layer thickness, usually about a few nm, which allows the application of the ideal Mott-Schottky equation. Thus, the present work analyzes the effect of a nanostructured semiconductor electrode, not so small with respect to the characteristic length scales (viz., depletion layer thickness and double layer thickness) of the interface, on interfacial electron-transfer simply on the basis of its surface energetics without the need for a complicated mathematical analysis to treat the mass transport phenomena. Accordingly, the complex plane impedance plots obtained by applying different magnitudes of dc voltage bias superimposed on a 10 mV rms ac signal at pH 4 are fitted with the equivalent circuit shown in Figure 1b, and the space-charge capacitance (Csc-2) obtained from the circuit is plotted against the magnitude of the applied dc bias (E). The resultant Csc-2 versus E data is fitted by a linear regression (Figure 7) using the Mott-Schottky equation (eq 3). As evident from Figure 7, the plot shows a linear behavior consistent with the MottSchottky expression for the space-charge capacitance. However, a shift in the intercept with the appearance of an intermediate region is observed in the Mott-Schottky plot, presumably signifying a surface-state-mediated mechanism for the chargetransfer process at the ZnO multipod/electrolyte interface which could be explained as follows.33,52 When there is a high density of surface states, the Fermi level of the semiconductor can become “pinned” within the potential range in which the surface states occur. In this potential range, the maximum potential drop occurs within the Helmholtz layer and the space-charge capacitance will not change much with the applied potential. As a result of this, the intercept at the potential axis is shifted from its true flat-band potential value. However, as the surface states become fully empty or fully occupied (depending on whether it is a donor state or an acceptor state), the capacitance starts to change again with applied potential. In this context, the capacitance-potential plot shown in Figure 7 indicates a surface-state-mediated mechanism for the interfacial chargetransfer processes. In addition, the slope of the intermediate region in the Mott-Schottky plot is reminiscent of the density of the surface states. For example, the higher the slope in the central region, the lower the density of states and vice versa. However, the negative slope in the central region observed in
Parthasarathy et al.
Figure 7. Mott-Schottky plot of the ZnO multipods at pH 4 obtained by fitting the frequency-dependent impedance data with a 10 mV rms ac signal superimposed on dc bias voltages varied in steps of 10 mV with an equivalent circuit based on a fluctuating energy level model. The shift in the slope of the Mott-Schottky plot shows the existence of surface states, and its finite magnitude shows their polyenergetic nature. The flat-band potential Vfb′′ is calculated to be -0.42 V versus Ag/AgCl from the intercept in the potential axis (obtained by scaling the capacitance axis from 0 to 1 × 1012), and a dopant density of 1.84 × 1017cm-3 is obtained from the slope.
the present case could be due to a distribution of surface states along the anisotropic multipod structures, thereby making them polyenergetic in nature, which also becomes evident when the potential dependence of the capacitance values is correlated with the pH dependence as discussed in section 4.2.3. The effect of shape anisotropy of the electrode on the distribution of surface states could be discussed from two main perspectives. One is its effect on the distribution of adsorption sites of the electroactive species along the anisotropic mesostructure and the other is its overlap with the polycrystalline nature of the multipods (as the energy of adsorption also depends on the exposed crystal face). Further, the flat-band potential obtained from the MS plot (-0.42 V vs Ag/AgCl) is ∼200 mV more negative than that reported by Fichou et al. for Fe3+ doped polycrystalline ZnO at pH 453 indicating a higher doping density51 in the ZnO multipods compared to the latter case (actually, Fichou et al. have reported a flat-band potential of -0.24 V vs SCE for Fe3+ doped ZnO at pH 7; since a linear dependence between Efb and pH is reported, a simple calculation yields an Efb value of -0.14 V vs SCE at pH 4). Nakabayashi et al. have reported a similar shift in Efb after photosensitization of dye materials adsorbed on ZnO single-crystal surface,54 which also seems to be the effect of surface-state formation. However, these authors seem to have overlooked the formation of surface states on the basis of a report by Kurtin et al. about the impossibility of formation of surface states on ionic semiconductor crystals.55 A closer examination of the latter report55 shows that the particular conclusion deals exclusively with Tamm states at a semiconductor/vacuum interface (i.e., surface states formed by abrupt changes in lattice periodicity at the semiconductor/vacuum interface) and not with plausible surface states formed because of the adsorption of ionic species at a semiconductor/electrolyte interface (Shockley states). On the basis of the above argument, the results obtained by Nakabayashi et al., when interpreted on the basis of surface states, sound similar to the results obtained in the present investigation as follows. The energy level of the dye in the oxidized form obtained on photoexcitation becomes closer to the conduction band edge than the energy level of the reduced form present before photoexcitation and subsequently
Electron Transfer at Nanostructured ZnO Multipods results in shallow surface states (as evident from the dependence of the shift in Efb on the intrinsic redox potential of the dye molecules). The above discrimination is brought into notice in this report to exhibit the undetermined similarity in the approach with our methodology, involving a variation in the solutionphase redox potential by varying the nature of the dye molecule. On the other hand, an Efb of -0. 75 V (vs SCE) has been reported for polycrystalline ZnO films on Zn electrode formed electrochemically in 0.1 M NaOH, and a value of -0.4 V has been reported for single crystalline ZnO at pH 7. However, in the above comparisons, the band gap of nanostructured ZnO is considered to be the same as that of bulk ZnO (∼3.3 eV) as the dimension of the multipods is well above the excitonic Bohr radius of ZnO (2.06 nm) on the basis of effective mass approximation (EMA).56,57 In addition to the above observation, the frequency dependence of the calculated space-charge capacitance confirms the adsorption of ferricyanide species on the electrode surface. This could be compared with a report by Hamann et al. in which a frequency-independent space-charge capacitance for a series of nonadsorbing redox couples on single crystalline ZnO has been mentioned along with an analysis of the free-energy dependence of electron-transfer rates using the Marcus theory.30 Further, a dopant density of 1.84 × 1017 cm-3 is obtained from the slope of the Mott-Schottky plot, which is 1 order of magnitude higher than that reported for single crystalline ZnO at pH 8.4 and is 3 orders lower than that for polycrystalline ZnO films.58 However, dopant density of ZnO films is always 4 orders of magnitude higher than that of single crystalline ZnO owing to the variation in the stoichiometry of Zn1+δO.58,59 In the present case, the dopant density is intermediate between that for single crystal and that for polycrystalline films, which shows lesser surface defects in the ZnO multipods than that in polycrystalline films. However, the effect of surface geometry of the ZnO multipods on the capacitance values could be neglected as the dimensions of the multipods are considerably greater than the characteristic length scales of the interface, namely, the depletion layer thickness and the double layer thickness.60 Scheme 1 depicts the mechanism of surface-statemediated electron transfer at a semiconductor/electrolyte interface. This observation serves as a strong evidence for the validity of the equivalent circuit on the basis of the fluctuating energy level model employed for fitting the above impedance plots. 4.2.2. pH Dependence of Frequency Response. Having analyzed the effect of applied potential on the frequency dependence of impedance, it would be interesting to investigate the effect of electrolyte pH on the frequency response and capacitance values. As the electrolyte pH is varied systematically, the level of protonation of the ZnO surface could be controlled with a subsequent effect on the double layer structure. More specifically, the variations in surface charge after protonation is expected to affect various interfacial processes like the adsorption of electroactive species and hence the distribution, occupancy, and reactivity of surface states (thereby affecting the surface-state capacitance) in addition to its effect on the kinetics of interfacial electron transfer (thus affecting the frequency response). Thus, the surface states are expected to be more sensitive to variations in the electrolyte pH than the applied potential. On the other hand, the pH variation is also expected to affect the space-charge capacitance as the extent of band bending at the semiconductor/electrolyte interface depends on the redox potential of the electrolyte species. Accordingly, a second set of parameters pertaining to the surface states is derived from the pH dependence of frequency
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13099 SCHEME 1: Electron Transfer from the Conduction Band of an n-Type Semiconductor Electrode to the Acceptor Species in Solution via Polyenergetic Surface States Formed at the Semiconductor Surface Which Differs from Direct Tunneling of Electrons through the Potential Barrier in the Space-Charge Layera
a EF is the Fermi level of the semiconductor; EV corresponds to the valence band edge energy level; EC corresponds to the conduction band edge; Eredox is the Fermi level of the electroactive species in solution; and Eox and Ered are the energy levels of the oxidized and reduced forms of the electroactive species, respectively.
Figure 8. Nyquist plots (-Z(Im) vs Z(Re)) of ZnO multipods in 0.1 M KCl, with an equimolar mixture of K4[Fe(CN)6] and K3[Fe(CN)6] (5 mM each) in electrolytes of different pH’s (3.5, 3.9, 4.4) obtained by using a 10 mV rms AC signal superimposed on a dc voltage equal to the open circuit potential at each pH.
response. The pH range is chosen in such a way that the ZnO structures are considerably stable within the experimental time scale. Although ZnO is known to dissolve in acidic media,39 appreciable dissolution has been reported to occur only below a pH of 2 provided an anodic bias of at least 2 V is applied. Conversely, the minimum pH employed in this work is not less than 3.5 with bias voltages not exceeding 200 mV (vs Ag/AgCl). In addition, compared to the time scale of the experiments, the actual rate of dissolution is too small.39 However, the stability of ZnO multipods in the employed pH range is confirmed from the SEM images of the multipods recorded after dipping them in solutions of pH 3.5, 9, and 10.5 (Figure 4, Supporting Information). Thus, Figure 8 shows the complex plane impedance plots at different pH’s (3.5, 3.85, 4.43) from which surface state, Css, and space-charge capacitance, Csc, values are obtained by fitting the high-frequency part (150 Hz to 5 kHz) with the
13100 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Parthasarathy et al.
Figure 9. Variation of space-charge capacitance with pH of the electrolyte medium. The maximum at pH 4.5 corresponds to the pzzp of ZnO multipods in contact with the aqueous electrolyte.
equivalent circuit shown in Figure 1b. When the space-charge capacitance thus obtained is plotted against pH, a maximum is observed at pH 4.5 as shown in Figure 9. The pH at which the space-charge capacitance is maximum could be considered actually as the point of zero-zeta potential (pzzp) of the ZnO multipods in contact with the aqueous electrolyte. Also, this is in considerable agreement with the position of maximum in the OCV-pH plots mentioned above (Figure 5), demonstrating the striking difference in the point of zero-zeta potential (pzzp) of nanostructured ZnO from that reported for the bulk material (pH 9.5).61 Such a drastic decrease in pzzp could be explained on the basis of a lowering in surface potential and hence an increased reactivity of the nanomaterial in light of the following correlation:62
Vpzc ) (χM - χS) + [(µeR - µeM)/F] + S∆Rφ
(4)
where Vpzc is the potential of zero charge, χM and χS are the surface potentials of the metal electrode and solution each with respect to vacuum level, µeR and µeM are the chemical potentials of electrons in the reference and metal working electrodes, respectively, and S∆Rφ is the inner potential difference at the reference electrode/electrolyte interface. Subsequently, the potential at the metal/electrolyte interface, M∆Sg, is given in terms of the surface potentials χ by
∆Sg ) χM - χS + δχ
M
(5)
where δχ ) δχM - δχS takes into account the changes in the orientation of the solvent dipoles at the electrode/electrolyte interface. The remarkable pzzp shift could indeed be very useful as an appropriate quantitative scale for measuring the enhancement of reactivity during a transition from bulk to nanoscale. Another interesting consequence of this observation is that the reactivity at the semiconductor/electrolyte interface could be easily correlated with that at the semiconductor/vacuum interface as the surface potential, χM, of an electrode material is directly related to its work function, ΦM, as given by the expression30
ΦM ) -µeM + FχM
(6)
Strictly speaking, the ejection of electrons from an electrode is mainly dependent on the work function of the electrode material irrespective of whether the electron is released into
Figure 10. Dependence of faradaic resistance, Rct (charge-transfer resistance), with the electrolyte pH. Rct is obtained by fitting the electrochemical results with an equivalent circuit based on fluctuating energy level model using the relation Rct ) (R-1 + RL-1)-1.
vacuum or it is donated to an acceptor species in solution. The electrode/electrolyte interface differs from an electrode/vacuum interface only in terms of additional contributions from changes in orientation of the solvent dipoles and relative position of energy levels of dissolved acceptor species with respect to the energy levels in the electrode material. The depth of surface states and hence the electron capture cross section (i.e., the probability of electron transfer at the interface) are considerably affected by the nature of the electrolyte because the redox potential of dissolved acceptor species is a function of the electrolyte parameters such as pH and ionic strength, especially when the electrode material is a semiconductor. More specifically, the same material showing a drastic shift in pzzp in the present study is also shown to have enhanced field emission characteristics in our earlier study, thereby supporting the above argument. Further, the redox activity of the ZnO surface is found to be a minimum at the pzzp as evident from the plot of chargetransfer resistance, Rct, against pH shown as in Figure 10 leading to an interesting and remarkable demarcation between the semiconductor/vacuum interface and the semiconductor/electrolyte interface. While in the former case, the field emission current density increases with a decrease in the work function, the rate of electron transfer from the semiconductor to the ionic acceptor species, [Fe(CN)6]3-, is minimum at the pzzp perhaps because of the strong dependence of ionic forces at the interface on the effective charge density of the semiconductor surface. A further calculation based on Rct and Csc yields a value of ∼4.2 ms as the mean relaxation time of occupancy of the polyenergetic surface states near the isoelectric point of the multipods (pH 4.4) with respect to which it could be classified as a fast surface state as reported by Myamlin and Pleskov.63 4.2.3. Space Charge Versus Surface State. Analysis of impedance behavior with respect to the applied dc bias and electrolyte pH carried out so far provided considerable insight into the dynamics of processes occurring at the ZnO multipod/ electrolyte interface. However, we have yet to understand the actual nature of surface states because of considerable overlap with the space-charge effects. To aid a deeper understanding of the surface states, a correlation of the capacitance values obtained from the potential-dependent frequency response with those obtained from the pH-dependent frequency response might be more appropriate in the light of the expected sensitivity of surface states to electrolyte pH, as discussed in the previous
Electron Transfer at Nanostructured ZnO Multipods
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13101 electrolyte interface. We believe the present treatment to be very helpful to understand the energetics and dynamics of surface states formed because of the adsorption of species on semiconductor nanocrystals notwithstanding few limitations such as the negligence of the effect of surface inhomogeneities on electric field perturbations. 5. Summary and Conclusions Surface-state-mediated electron transfer of ferricyanide ions adsorbed at nanostructured ZnO multipods/aqueous electrolyte interface has been investigated by modeling the system with equivalent circuits developed on the basis of fluctuating energy level model. The polyenergetic nature of the surface states is obvious from the plot of space-charge capacitance from the above model against the applied dc bias. In addition, MottSchottky analysis of the capacitance-potential plots implies a positive shift in ZnO flat-band potential by 80 mV for the multipod/electrolyte interface compared to the bulk ZnO electrode/electrolyte interfaces, which is attributed to the presence of surface states. Interestingly, the drastic decrease in pzzp observed from pH dependence is in agreement with the attractive field emission characteristics of the material. A three-dimensional mapping of surface-state distribution is attempted by a correlation of pH dependence and potential dependence of spacecharge capacitance. Although the present work deals with only majority carrier processes, it is possible to correlate the results with minority carrier processes of the same material using a quasi-Fermi level approach. Here, ferricyanide/ZnO is employed only as a model system to demonstrate the importance of surface states in nanostructured semiconductor electrodes. However, the results could be extended to other semiconductor electrodes to treat the adsorption of a variety of organic molecules used as photosensitizers to understand interfacial charge-transfer dynamics in dye-sensitized ZnO solar cells and to improve further their photoconversion efficiencies. Acknowledgment. Meera and Bhaskar would like to acknowledge the Council of Scientific and Industrial Research (CSIR), India, for awarding a Senior Research Fellowship and Junior Research Fellowship, respectively. We thank Mr. A. B. Gaikwad for his valuable help in SEM measurements.
Figure 11. (a) Three-dimensional variation of space-charge capacitance with dc voltage superimposed on a 10 mV rms ac signal and with the pH of the electrolyte medium. (The color scale indicates the variation in capacitance.) (b) Three-dimensional variation of surface-state capacitance with dc bias superimposed on a 10 mV rms ac signal and the electrolyte pH. Multiple protrusions corresponding to polyenergetic surface states formed because of the irreversible adsorption of ferricyanide species on ZnO multipods are obvious from the plot.
Supporting Information Available: CV and impedance analysis of ZnO free Pt electrode; SEM images of ZnO multipods after treatment with pH 3.5, 9, and 10.5 solutions; FTIR spectrum showing the adsorption of ferricyanide species on the ZnO surface; impedance fitting parameters and sensitivity of fits to some parameters of interest in terms of Bode (φ versus log f) plots. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
section. Accordingly, Figure 11a and 11b shows the variation of space-charge capacitance and surface-state capacitance with pH and applied dc bias, respectively, in a three-dimensional surface. These plots could be viewed as an exquisite mapping of the surface states by coupling perturbations invoked from the electrolyte side to that applied from the electrode side. More specifically, the spatial variation of surface-state density is clearly evident from a comparison of the plots as the surfacestate capacitance shows multiple dips in the 3D surface in contrast to the variation of the space-charge capacitance as a function of such perturbation. This could be considered as a unique way of “mapping” the surface states at a semiconductor/
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