Article pubs.acs.org/JPCC
Charge Transfer across the n‑Type GaN−Electrolyte Interface Susanne Schaf̈ er, Amelie H. R. Koch,† Alda Cavallini, Martin Stutzmann,* and Ian D. Sharp*,‡ Walter Schottky Institut, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany ABSTRACT: The interfacial charge transfer characteristics of n-type GaN are investigated in pure phosphate buffered saline, as well as in solutions containing I−/ I3− or hydroquinone/benzoquinone redox couples. Cyclic voltammetry and transient photoresponse measurements in the presence of above-bandgap illumination reveal that hole transfer to the solution is mediated by surface states in all cases. For measurements in pure PBS, a modification of the surface during cyclic potential sweeps is observed. In contrast, the presence of the redox species used in this work efficiently suppresses the oxygen evolution reaction and the associated surface modification. Furthermore, charge transfer to the redox couple is fully reversible using GaN as a dark cathode and photoanode, respectively. The presented study is of significant importance for applications of GaN in photocatalysis and biosensing, where the stability of (bio)functionalized surfaces is an essential requirement.
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INTRODUCTION Gallium nitride (GaN) has attracted significant attention as a semiconductor for applications including light-emitting diodes,1 high power/temperature devices,2 and short-wavelength photodetectors.3 Due to its biocompatibility and stability in harsh environments, this material has also been employed for gas- and biosensing.4,5 Consequently, elaborate mechanisms for biofunctionalization of its surface have been developed6−10 and charge transfer in these organic/semiconductor hybrid systems has been investigated.11 Furthermore, due to the energetic positions of its band edges,12,13 GaN has also been discussed as a suitable electrode for water splitting.14 Although its wide bandgap of 3.4 eV severely limits its maximum efficiency under solar illumination, this material provides a model system for the study of semiconductor−electrolyte interfaces. More recently, the GaN:ZnO solid solution has been reported as a powerful photocatalytic material for electrolysis.15−17 In separate but related work, the electrochemical etching of GaN has been investigated for device processing applications, with a focus on photoanodic decomposition, oxidation, and etching in acidic and basic solutions.18,19 In this context, it was reported that Cl− ions in solution stabilize n-type GaN surfaces against photodecomposition and a mechanism for interfacial charge transfer of photogenerated holes via intrinsic surface states was proposed.20,21 Chakrapani et al. provided further spectroscopic evidence of the charge transfer between the oxygen redox couple in an adsorbed water film and GaN via midgap states.22 However, a thorough understanding of both charge transfer energetics and kinetics at the n-type GaN−electrolyte interface, which is required for applications in biosensing and photocatalysis, remains incomplete. In this work, we present a study of n-type GaN photoelectrodes in phosphate bufferd saline (PBS), as well as in PBS containing a redox couple (I−/I3− or hydroquinone/benzoquinone), using cyclic voltammetry. Furthermore, transient photocurrent and open circuit potential measurements are used to elucidate the contributions of surface © 2012 American Chemical Society
states in interfacial charge transfer processes. These results reveal that midgap states play a critical role in mediating interfacial charge transfer and that physical and chemical surface instabilities can be kinetically mitigated by introduction of appropriate redox couples.
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EXPERIMENTAL SECTION Metal−organic chemical vapor deposition (MOCVD) grown (0001) n-type GaN samples (silicon donor concentration ND = 2 × 1018 cm−3) were purchased from Lumilog. Ohmic top contacts (Ti/Al 30/80 nm) were evaporated onto the sample corners and annealed at 500 °C for 5 min in a vacuum.23 Prior to measurements, all samples were cleaned in acetone in an ultrasonic bath for 10 min, rinsed with isopropanol, and dried with flowing nitrogen. The samples were mounted in a PEEK holder, which exposed a circular active electrode area of 0.28 cm2 and shielded the contacts, as well as the electrical wiring, from the electrolyte. All measurements were performed in a three-electrode electrochemical setup in a UV-transparent quartz glass beaker with a Ag/AgCl electrode as the reference electrode and a platinum wire as the auxiliary electrode (Metrohm AG). Cyclic voltammetry and amperometry were performed with a Gamry Reference 600 potentiostat. Potassium-based phosphate buffered saline solution (PBS, 10 mM, ionic strength 100 mM) was used as the electrolyte. The buffer was adjusted to pH 7.0, and N2 gas was bubbled through the solution for at least 2 h prior to measurement to remove dissolved oxygen. To this aqueous electrolyte, 0.5 mM I−/I3− or 1 mM hydroquinone (HQ) was added for the measurements in redox solution. All cyclic voltammograms were collected with a scan rate of 250 mV s−1, if not stated otherwise. The 254 nm emission from a 4 W Hg lamp was used for illumination of the Received: February 29, 2012 Revised: September 13, 2012 Published: October 1, 2012 22281
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on a complex set of factors including the surface state concentration and occupation, which affect the magnitude of the surface band bending at a given applied potential, the recombination of photogenerated carriers that competes with interfacial charge transfer processes, and specific catalytic properties of the surface. In the present case, the onset of the anodic current in the reverse scan direction occurred at −0.8 V and saturated to a maximum value of 6.7 μA cm−2 at potentials more positive than −0.2 V. The magnitude of the saturated photocurrent was dependent on the incident light intensity and, hence, the concentration of photogenerated free holes (data not shown). Illumination with 366 nm resulted in qualitatively very similar spectra with respect to anodic current onset and saturation bias. Due to the lower incident power of abovebandgap light, the maximum anodic current was decreased to 2.2 μA cm−2. The similarity of the spectra at different wavelengths confirms that direct abstraction of OH-groups by 254 nm illumination can be eliminated as a significant factor in the measured photoelectrochemical behavior. Previously, such an anodic photocurrent was attributed to the oxygen evolution reaction (OER) due to transfer of photogenerated free holes to the O2/OH− redox couple in solution, mediated by surface states,18 or to photo-oxidation of the GaN surface.24,25 In the present work, a strong hysteresis between the forward and reverse scan directions was observed in the anodic region of the spectrum under illumination. The observed anodic hysteresis could be caused by either an altered occupation density of surface states, which would cause a shift of the band bending between forward and reverse scans, or a physical or chemical modification of the surface in the harsh environment associated with the O2/OH− redox reaction, as will be addressed later in this work. Within the first few cyclic potential sweeps, a broad cathodic peak evolved with a maximum at −1.38 V and a shoulder at −1.0 V. This peak was only observed following anodic sweeps under illumination, and its magnitude was correlated with the total anodic current. Accordingly, it is attributed to the reduction of oxygen which was generated at anodic potentials under illumination. The origin of the cathodic contribution appearing as a shoulder at −1.0 V is not clear at present but is likely related to the reduction of surface states. Transient photocurrent measurements were performed in order to investigate the contributions of surface states to the charge transfer process and to determine their role in the observed anodic hysteresis. At a sample bias of +0.4 V, the photocurrent immediately reached its saturated level and no differences were observed for transients recorded after sweeping from the cathodic (*) and anodic (◆) potential range (Figure 1b). In contrast, for sample biases within the hysteresis region (−0.4 and −0.2 V), sharp anodic current spikes were observed when the light was switched on. These anodic spikes decayed exponentially with a time constant t1, on the order of milliseconds. Interestingly, both the steady state photocurrents and the recorded transients at (*) and (◆) differed significantly, reflecting the magnitude of the observed hysteresis at a given bias. While the transients recorded at (◆) quickly reached a steady state, the transients at (*) exihibited a second, much longer time constant, τ2, on the order of seconds. In this case, the photoresponse did not reach steady state until several illumination cycles over a period of approximately 20 s. Sharp anodic spikes in other semiconductors have been reported to result from photogenerated holes accumulating at the interface, due to either slow oxygen reaction kinetics or the oxidation of trap states (Figure 2).26 The integrated area, A,
samples with above-bandgap light. In addition, reference measurements were performed in PBS using the 366 nm emission of the same Hg lamp, providing light of significantly lower energy. Due to the finite width of the spectral line, a small fraction of the 366 nm illumination provides light with energy just above the GaN bandgap. Transient photoresponse curves were measured by recording the time-resolved photocurrent at a fixed sample bias while switching the light on and off with a computer-controlled camera shutter. The transient measurements were repeated twice for each sample bias after stopping the preceding cyclic voltage scan at the desired bias, approached from the cathodic (marked by *) or the anodic region (marked by ◆) of the spectrum. To investigate the GaN surface band bending as a function of the electrolyte redox potential, open circuit potential measurements were performed in PBS, and in PBS containing I−/I3− (0.3 mM). Changes of the sample morphology due to electrochemical treatment were investigated by atomic force microscopy (AFM) (Veeco, Nanoscope V), using Si cantilevers in tapping mode.
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RESULTS AND DISCUSSION N-type GaN without additional illumination is a cathode. Accordingly, no current was measured for biases more positive than −1.0 V, which corresponds to the flatband potential, Vfb, of n-type GaN in PBS solution.13,19 For sample biases more negative than Vfb, a cathodic current was observed, which was attributed to proton reduction to H2 via electron transfer from the conduction band into the electrolyte (Figure 1a). Under above-bandgap illumination, n-type GaN acts as a photoanode (Figure 1a). Generally, the onset of the anodic current depends
Figure 1. (a) Cyclic voltammograms of n-type GaN in neutral PBS in darkness (dashed), under above-bandgap illumination with 254 nm (solid), and with 366 nm (dash-dot). (b) Transient photoresponse at fixed sample biases (indicated in (a)), approached from the cathodic (*) or anodic (◆) side of the spectrum. τ1 and τ2 denote the time constants associated with charging of surface states and modification of the GaN surface, respectively. 22282
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Figure 2. Phenomenologic scheme of n-type GaN in (redox) solution. The magnitude of band bending depends on the chemical potential of the solution, the applied sample bias, and the surface state concentration. Solid arrows indicate photogeneration and separation of charge, while dashed arrows indicate the hole transfer into the solution via surface states, S, under above-bandgap illumination. Figure 3. Open circuit potential of bare n-type GaN for several illumination cycles (254 nm) in PBS (a) and I−/I−3 (b).
between the recorded photocurrent and a square wave (if the photocurrent had immediately reached steady state) yields an estimate of the accumulated hole density at the interface under the specific illumination conditions (Table 1). In PBS solution,
contrast to the case in PBS solution, no oxygen reduction peak was detected. Within several cycles, the cathodic peaks increased to their current density maxima of 8.5 μA cm−2 (15 μA cm−2) for HQ/BQ (I−/I3−). Once the light was switched off, the cathodic peaks decreased within a few cycles (Figure 4b,e), as the oxidized redox species were consumed. For alternating measurements in darkness and under illumination, the recorded cyclic voltammograms were highly reproducible, using GaN alternately as a dark cathode and photoactivated anode, respectively. The oxidized species generated under UV-illumination (BQ and I3−) are reduced at potentials more negative than Vfb. Consequently, the reduction peak maximum is expected to depend on the total quantity of oxidized species formed during the anodic sweep. Indeed, the maximum cathodic current density was dependent on both the anodic limit of the applied sample bias (shown for HQ/BQ in Figure 4c) and the applied scan rate (shown for I−/I3− in Figure 4f). An approximately linear correlation was found between the magnitude of the applied anodic bias and the maximum cathodic current density, when the anodic bias limit was set higher than the onset potential of the photocurrent (inset Figure 4c). In contrast, the dependence of the maximum current on the scan rate followed a power law behavior, indicating a stronger limitation by diffusion (inset Figure 4f). Since no reduction of an anodic OER product was observed during the cathodic sweep, the anodic photocurrent is assigned to hole transfer onto the reduced component of the available redox couple. It is reasonable to assume that charge transfer to a redox couple is kinetically favored and thereby suppresses the slow oxygen evolution reaction.19 As in PBS, hole transfer is expected to be mediated by surface states, since the redox levels are energetically higher than the O2/OH− level (Figure 2). The shoulder observed in the cathodic region is likely associated with the reduction of surface states. Remarkably, no hysteresis is observed for cyclic voltammograms in redox solution for scan rates as low as 50 mV s−1. Although this could be caused by a different occupation of surface states by, for example, surface passivation by redox ions, suppression of the OER could also render the local environment less harsh and inhibit physical or chemical surface modification.
Table 1. Densities of Accumulated Photogenerated Holes Extracted from Transient Photoresponse Curves at Fixed Sample Biases, Ubias solution PBS
PBS, I−/I3−
PBS, HQ/BQ
Ubias (V)
area (μC cm−2)
+0.4 −0.2 −0.4 −0.4 −0.6 −0.7 −0.4 −0.6
0.0 0.32 0.38 0.0 0.18 0.10 0.0 0.13
hole density (cm−2) 2.0 × 1012 2.4 × 1012 1.1 × 1012 0.6 × 1012 0.8 × 1012
a value of approximately (2−2.5) × 1012 cm−2 accumulated holes was determined from the transient photoresponse curves obtained at potentials within the hysteresis region. The precise density depends on the illumination intensity, the applied sample bias, and the corresponding surface band bending, which provides the driving force for drift of photogenerated holes to the surface. The second time constant (τ2) is too large to be caused by recombination at trap states but rather suggests physical and/or chemical modification of the surface. Cyclic voltammograms were acquired in PBS solutions containing hydroquinone (HQ) or I−/I3− redox species. When holes are provided to the hydroquinone molecules in solution, benzoquinone (BQ) can be generated, forming the HQ/BQ redox couple. Under above-bandgap illumination, the anodic photocurrent saturated at 1.6 and 2.5 μA cm−2 for HQ/BQ and I−/I3−, respectively (Figure 4a,d). The strong decrease of the saturated photocurrent in redox solution is due to the enhanced UV-absorbance of the solution. Pronounced cathodic peaks were observed at −1.19 V (−1.08 V) for HQ/BQ (I−/I3−), which are in good agreement with the expected differences of electrochemical potential of these redox couples relative to the OER and the corresponding shift in GaN surface band bending. For both redox species, a second cathodic contribution was resolved, shifted by 0.25 and 0.2 V to lower cathodic potentials for HQ/BQ and I−/I3−, respectively (Figure 4 (○)). In 22283
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Figure 4. Cyclic voltammograms for n-type GaN in neutral PBS containing HQ/BQ or I−/I3− redox couple. Arrows indicate the evolution of the reduction peaks under above-bandgap illumination (a, d) and in darkness (b, e). The peak associated with reduction of surface states is marked by (○). Dependence of the maximum cathodic current density on the applied anodic bias limit ((c) and inset) and the scan rate ((d) and inset).
where the SPV in I−/I3− was weighed with a factor of 1.3 to account for the higher UV absorption in the redox solution. For a completely unpinned Fermi level, a ratio of unity would be expected. The obtained ratio of 0.8 predicts an incomplete pinning of the n-type GaN Fermi level, allowing shifts upon variations in the redox potential to a large extent. Notably, the recorded transient behavior in I−/I3− solution upon switching off the light was much faster compared to PBS, providing evidence for the enhanced transfer kinetics in the presence of redox species in solution. Again, transient measurements were performed at fixed sample biases to elucidate the role of surface states in the charge transfer (Figure 5b). The decay time constant of the recorded anodic spikes was in the range of milliseonds and hence comparable to τ1 determined for PBS. Additionally, the density of accumulated holes was calculated to be 0.8 × 1012 and (0.6−1.1) × 10 12 cm−2 for HQ/BQ and I−/I3−, respectively. Considering the differences in UV absorption for the different solutions, the obtained values are comparable to PBS. The presence of anodic spikes in both PBS and redox solution confirms oxidation of surface states as the origin of the anodic spikes rather than slow water oxidation kinetics, which only apply for PBS. Furthermore, this supports the assignment of the observed shoulder in the cathodic peak to the reduction of surface states. In contrast to the transient measurements performed in PBS, those recorded in the presence of HQ/BQ or I−/I3− were identical regardless of whether the fixed sample bias was approached from the anodic or cathodic side of the spectrum. No long time constant, τ2, was observed, suggesting that this quantity and the resulting anodic hysteresis in pure PBS are associated with modification of the surface rather than a slowly varying occupation of surface states. On the basis of these observations, it was tentatively concluded that the presence of redox species in solution stabilizes the GaN surface against modification. The stabilizing effect of redox ions in solution was confirmed by AFM (Figure 6). For solvent cleaned, n-type GaN, the
The observed shifts in cathodic peak position in the cyclic voltammograms were related to the required alignment of the GaN Fermi level with the redox potential in solution at equilibrium, determining the interfacial band bending. When the Fermi level is aligned with the electrochemical redox potential, the resulting upward band bending is higher for an alignment with E0(HQ/BQ) than for E(I0 −/I3−), and is yet higher for 0 alignment with E(OH − /O2). The GaN conduction band position is energetically far above the redox levels. Therefore, majority electrons are expected to possess sufficient energy to drive all three reactions. In order for electrons to be available at the surface, potentials more negative than the flatband potential must be applied. For an unpinned Fermi level, a decrease in surface band bending and thus in overpotential for the reduction reaction is expected following the trend VOH−/O2 < VHQ/BQ < VI−/I3− . However, the contribution of surface states visible in the cyclic voltammograms suggests that the Fermi level could be partially pinned. To this end, open circuit potential measurements were performed for n-type GaN in PBS and I−/I3− (Figure 3). The first illumination cycle on ntype GaN in PBS revealed a surface photovoltage (SPV) of 0.8 V. This value is smaller than the magnitude of the flatband voltage (1.23 V), which was reported by G. Steinhoff et al.,13 indicating incomplete band flattening by the employed Hg emission. After switching off the light, the transient behavior was slow, confirming charge trapping in surface states. For subsequent illumination cycles, the recorded SPV was decreased to 0.33 V, as expected for a strong contribution of charged surface states. In I−/I3− solution, an SPV of 0.44 V was recorded, which remained stable for several illumination cycles. Consequently, the SPV (and the interfacial band bending) were shifted with the redox potential in solution (Figure 2), according to dSPV 0.8 − 0.44 × 1.3 = = 0.8 0 0.82 − 0.53 dEredox
(1) 22284
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roughness of 0.45 nm (Figure 6b). In contrast, for measurements in redox solution, the surface structure and the determined rms roughness remained unaltered (Figure 6c,d). However, for the case of I−/I3−, the sharpness of the growth steps is lost, providing some indication of early stage surface modification. This observation is in accordance with the final cyclic potential sweep measurements on this particular sample at low scan rates, which exhibited the onset of a hysteresis. Thus, the described mechanisms were verified by AFM, highlighting the importance of our results for the application of (bio)functionalized n-type GaN surfaces in aqueous solution. Notably, the discussion in this work only considers welldefined, highly doped, n-type GaN as a model system for interfacial charge transfer in redox solution. Although comparative measurements on p-type GaN were performed, the unresolved material issues, which lead to a sluggish and complex charge transfer behavior with low reproducibility, place these measurements outside the scope of this work.
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CONCLUSION Cyclic voltammetry, transient photocurrent, and open circuit potential measurements were utilized to investigate the interfacial charge transfer characteristics of n-type GaN in neutral PBS solution, as well as PBS containing redox couples with chemical potentials close to the O2/OH− level. Under above-bandgap illumination in PBS, oxygen evolution occurred in the anodic region, while generated oxygen was reduced at cathodic potentials. Charge transfer was found to be mediated via surface states. Interaction of the n-type GaN with the electrolyte under conditions favorable for the OER caused a modification of the GaN surface, resulting in a strong hysteresis in the cyclic current-bias scans. The kinetically slow OER and the associated surface modification were efficiently suppressed by adding redox species to the solution, resulting in the absence of a hysteresis in the spectra. The GaN Fermi level was only moderately pinned, following changes in the electrochemical potential of the solution to a large extent. For both investigated redox couples, charge transfer was found to occur via surface states, and could be tailored in magnitude by altering illumination intensity, scan rate, or the anodic bias limit. The redox reactions were fully reversible using n-type GaN alternately as a dark cathode and photoanode, respectively. The results of the electrochemical measurements were supported by AFM micrographs. The presented study demonstrates that GaN provides a model platform for studying charge transfer at semiconductor−electrolyte interfaces. Furthermore, these results are of importance for applications of GaN in photocatalysis and biosensing, where the stability of (bio)functionalized surfaces is an important requirement.
Figure 5. (a) Cyclic voltammograms of n-type GaN in PBS, HQ/BQ, and I−/I3− solutions. The shifts of the reduction peaks are in accordance with their respective chemical potentials. (b) Transient photoresponse curves at fixed sample biases (indicated by the black arrows in (a)) in I−/I3−, approached from the cathodic (solid) or anodic (dashed) side of the spectrum. τ1 denotes the time constant associated with oxidation of surface states.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (M.S.);
[email protected] (I.D.S.).
Figure 6. AFM micrographs (5 × 5 μm2) of a solvent-cleaned reference sample (a), along with corresponding images from samples after electrochemical treatment in PBS (b), HQ/BQ (c), and I−/I3− (d). z-scale = 3 nm.
Present Addresses †
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. ‡ Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, USA.
typical surface morphology, including growth steps, was resolved and the rms roughness was determined to be 0.3 nm (Figure 6a). After the electrochemical measurements in PBS, the surface was clearly modified, with an increased rms
Notes
The authors declare no competing financial interest. 22285
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ACKNOWLEDGMENTS This work was supported by Deutsche Forschungsgemeinschaft (DFG) through the TUM International Graduate School of Science and Engineering (IGSSE). I.D.S. acknowledges support from the Technische Universität München - Institute for Advanced Study, funded by the German Excellence Initiative.
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