Control of Carrier Recombination on ZnO Nanowires

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Control of Carrier Recombination on ZnO Nanowires Photoelectrochemistry Pushpa Chhetri, Krishna K. Barakoti, and Mario A. Alpuche-Aviles J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5071067 • Publication Date (Web): 18 Dec 2014 Downloaded from http://pubs.acs.org on December 19, 2014

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The Journal of Physical Chemistry

Control of Carrier Recombination on ZnO Nanowires Photoelectrochemistry Pushpa Chhetri, Krishna K. Barakoti and Mario A. Alpuche-Aviles* Department of Chemistry, University of Nevada, Reno, Nevada 89557, USA Abstract Surface recombination on ZnO nanowires (NWs) dominates over photogenerated carrier collection unless band bending and electrochemical reactions facilitate charge separation. We studied acetonitrile electrolyte with tetrabutylammonium perchlorate, with and without redox mediators (ferrocene/ferrocenium, iodide/triiodide and p-benzoquinone, BQ0/−). Faradaic processes successfully compete with recombination under biases that cause band bending along the NW c axis. Redox processes control carrier recombination mainly by holes (h+) removal and this affects the NW stability. Without redox mediator, recombination controls the photoelectrochemical behavior unless h+ oxidizes oxide ions in the lattice. Iodide stabilizes the material because the rate of I− oxidation by h+ is faster than the decomposition of ZnO. The conduction band edge (EC) of NWs and of single crystals terminated on the (1 0 −1 0) planes that constitute most of the NW surface was determined to be EC = –0.3 ± 0.1 V vs NHE; the decomposition potential is estimated to be 3.7 V. The NW photocurrent onset is positive of EC because surface recombination is S ≥ 53 cm/s; S increases with surface hydroxide content. The 1

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surface states responsible for this recombination lie under EC and are proposed to result from NW surface reconstruction.

Keywords: Electrochemistry, Zincite, carrier recombination, surface states, carrier density

Introduction

We investigate the effect of structure and electrolyte composition on the collection and recombination of photogenerated carriers from ZnO nanowires (NWs) at the solid/liquid interface. Because of the interest in applications of ZnO materials in photovoltaic devices such as the dye-sensitized solar cells (DSSCs), we study the interdependence of faradaic processes and photogenerated carrier recombination in nonaqueous electrolyte solutions. Further, we propose the use of these NWs as a model study for the effect of structure and surface composition on the band structure and recombination rates. The development of synthetic methods of ZnO structures like NWs,1-4 nanorods5 and microtubes,6 has renewed interest in ZnO materials for solar energy conversion,7 e.g., as a support for dye-sensitized solar cells,8-11 quantum dots sensitized solar cells,12 and as a substrate for multiple exciton injection13 from quantum dots (QDs) recently demonstrated.14-15 ZnO was one of the earlier demonstrations of semiconductor sensitization (e.g., ref

16

and references therein) and it is currently used as a model substrate.17-20 These

optoelectronic applications are possible due to the electronic properties of ZnO: wide direct band gap (Eg) energy of 3.37 eV, large exciton binding energy of 60 meV as well as its very high 2


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electron mobility.21-22 ZnO NWs of single crystalline nature have been reported to have higher electron mobility than TiO2 nanoparticle (NP) films making ZnO an attractive alternative for DSSC support.11 The position of conduction band edge (EC) is of fundamental importance. Data for single crystals exists in MeCN,20,23 and in aqueous solution, the energetics of electron transfer from the ZnO conduction band (CB) have shown the inverted Marcus region.24 Jacobsson et al.25 have recently determined the absolute band edge positions of ZnO QDs in the quantum confined regime between 4 and 9 nm. They reported that the increase in optical band gap with decrease in particle size causes a shift in EC towards more negative potentials. For applications in photovoltaic devices and optoelectronics, knowledge of the position of the conduction band edge (EC) as well as of the recombination centers is important in order to understand the energetics and the limiting rates of charge carrier collection. Under electrochemical conditions, i.e., at the solid/liquid interface, the flat band potential (Efb) of the ZnO NWs cannot be approximated by the photocurrent onset (Eon) due to recombination losses. We find that photogenerated electrons (e−) and holes (h+) recombine on ZnO NWs and shift the main photocurrent onset potential from the conduction band edge for different redox couples. Charge carrier recombination is one of the main causes of efficiency losses in several solar energy conversion devices. Recently, carrier lifetime in ZnO NWs immersed in aqueous solutions was found to be pico-second to nanosecond and assigned to hole trapping by zinc vacancies while the electron-hole separation time scale was proposed to be in the 900 - 400 ps, depending on the electrochemical bias.26 Although surface state have been widely considered as facilitating recombination, recent studies on MoS2 indicate that surface states can also tune the electrocatalytic properties of the material.27 In this paper, we study the electrochemistry and photoelectrochemistry of ZnO NWs and we compare 3



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− their behavior to single crystal electrodes that are terminated on the plane (1 0 1 0); this plane constitutes most of the NW surface. We also compare the photocurrent behavior to NPs of 30 nm diameter, i.e., in the absence of quantum confinement, to survey the effect of shape on the electrode behavior.28 We use CH3CN to provide a large electrochemical window for our experiments23,29 and we find that the main photocurrent onset potential is positive in the electrochemical scale with respect to I−/I3− (lower energy in the vacuum scale) even in solutions containing iodide. We propose that this is due to a shift from Eon toward positive electrochemical potentials with respect to EC due to recombination losses and we use this shift to estimate the rate of surface recombination.30 We examine the electrochemical and photoelectrochemical behavior in terms of the surface recombination kinetics, redox properties and the stability of the ZnO material. Experimental methods Reagents and Materials. All the reagents used in the synthesis of ZnO are of analytical grades and used without purification: Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 99.5%, EM Science), Zn(NO3)2·6H2O, 98% (Baker Company), NaOH (98%), tetrabutylammonium iodide (TBAI, ≥99%) and ferrocene (98%) were purchased from Sigma-Aldrich, Milwaukee. Methanol (CH3OH, HPLC grade) was purchased from Pharmco-AAPER and p-benzoquinone from Acros. Polyethylene glycol (PEG, 20000) and tetrabutylammonium perchlorate (TBAP, electrochemical grade) were purchased from Alfa Aesar. TBAP was recrystallized in 95% ethanol prior to use. All aqueous solutions were prepared in 18 MΩ⋅cm (Barnstead nanopure, Thermo Scientific). Non-aqueous solutions for electrochemical measurements were prepared in acetonitrile (CH3CN, HPLC grade, Pharmco-AAPER) purified by incubating in activated Alumina (Al2O3, N-Super 1, MP Biomedicals, 18 mg Al2O3/ml of CH3CN), for at least 1 week in an Ar glove box. 4


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Material preparation. Full details are given in the supporting information (SI). Briefly, ZnO NPs of 30 nm average diam were grown by hydrothermal growth as reported previosly31 from zinc acetate precursor grown in basic solution. Films of NPs were prepared on fluorine-doped tine oxide (FTO) on glass using the doctor blade method. NWs were synthesized by modifying a procedure from Greene et al.1 This procedure allows us to grow the NWs on a seeded FTO surface using an aqueous Zn(NO3)2 solution adjusted to a pH 10.67 with ammonia water. The NWs were used (a) as prepared and (b) after heat treatment (400⁰C for 30 min in air). Pellets were prepared using the standard procedure of pressing a powder and sintering. ZnO single crystals were obtained from MTI Corp (Richmond, CA). The crystals were terminated on (1 0 −1 0) planes and were mounted as described in the SI, section II. Characterization. XRD measurements were performed (Philips, Cu Kα, λ= 1.5405 Å) to characterize ZnO NPs and NWs. ZnO NW films were characterized by SEM (Hitachi S-4700-II) and ZnO NPs by transmission electron microscopy (TEM, JOEL JEM-2100F). Electrochemical and Photoelectrochemical Measurements. Oxidation currents are shown as positive following the IUPAC convention. Experiments were performed with a Gamry Reference 600 potentiostat and with a CH Instruments (CHI 760D and 700D) work station. All electrochemical and photoelectrochemical experiments were carried out in a PTFE cell with three electrode configuration. The cell was filled and assembled in an Ar glove box and sealed for the experiments to be conducted on the bench. ZnO NWs/FTO, NPs/FTO or pellets were the working electrodes (WE), coiled Pt wire was the counter electrode (CE) and a home-made32 I−/I3− reference electrode (RE) made by inserting a Pt wire in 10 mM I−/I3− in CH3CN is used. The RE was always used with a cracked glass junction containing 0.1 M TBAP in CH3CN. All the potential measurements were taken with respect to this 10 mM I-/I3- RE unless otherwise 5



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mentioned. This RE was calibrated using the ferrocene (Fc)/ferrocenium (Fc+) couple (E0’ = -0.2 V vs Fc/Fc+ and 0.4 V vs NHE). The electrolyte solution used for Efb calculation was 0.1 M TBAP in CH3CN. The experiments carried out to study the effect of I− ion, TBAI solution was added to the 0.1 M TBAP in CH3CN to make the overall concentration 1.94 mM. The area of working electrode was 0.64 cm2 defined by a PTFE-encapsulated O-ring (McMaster-Carr). The closed cell for photoelectrochemical measurement (assembled in Ar glove box) was used outside the glove box in front of a Xe arc lamp (150 watt, Oriel-Newport). A shutter (built in house) was used for chopping the light for periods of 10 seconds in the dark and 10 seconds of illumination. The electrochemical potential scan rate of 1 mV/s was used in chopped light experiments, unless otherwise noted. A water IR filter (Newport) was used to minimize the thermal agitation to the system. Mott-Schottky (MS) data were collected using the built-in impedance analyzer of the Gamry Reference 600 potentiostat. The data was collected under potentiostatic control at 1000 Hz in the dark, as previously reported.20 Mott-Schottky plots were collected at three different frequencies in 0.1 M TBAP to verify that they were independent of frequency (see SI, Figure S1).

Results and Discussion Material Characterization. XRD measurements of ZnO NP films and NWs grown on FTO match the crystallographic phases of wurtzite structure of zincite. For NPs (Figure 1a) the relative intensity of all the indexed peaks matches that of the zincite database (PDF # 36-1451, Figure 1c) without evidence of other crystallographic phases present. For ZnO NWs the peak for plane (002) is larger than expected from the database (Figure 1b) due to the preferential growth of nanowires along the c-axis.1-4 The scanning electron microscopy (SEM) images of NWs in 6


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Figure 2a shows the hexagonal structure as well as their vertical alignment with thorough coverage of the FTO surface (Figure 2b). For the NP films, the average particle size measured from TEM micrographs (Figure 2c) is 30 ± 3 nm. From XRD peaks for the (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes using Scherrer equation, the NPs crystalline domain is 26 ± 3 nm in line with the TEM images and consistent with the NPs being single crystals. Further evidence from the crystalline nature of the NPs comes from Figures 2d and 2e that show high resolution TEM for one NP where the fringe spacing of ≈0.26 nm of the ZnO (002) plane is visible. 12000

a)

a) 8000

Intensity (a.u)

4000

0

b) 30000 15000

*

*

*

* (103)

50

c)

(200) (112) (201)

(102)

100

c)

b) b) (110)

c)

(101)

0

(100) (002)

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0 30

40

50

60

70

2θ (degrees)

Figure 1. Powder X-ray diffraction of a) ZnO NP, 30 nm diam. b) ZnO NWs on FTO with the SnO2 peaks (*) c) Zincite syn, pattern, PDF#36-1451. 7



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Figure 2. Electron microscopy images of ZnO. (a) NW on FTO from the top and (b) side view (c) TEM of NPs of 30 nm diameter, scale bar is 1 µm on (a) and (b); (d) FFT of region (e) and (e) high resolution TEM on NP.

Mott-Schottky Experiments. We performed Mott-Schottky (MS) experiments with the single crystal electrodes terminated on the (1 0 −1 0) planes that make most of the NWs surface. Figure 3 shows the experiments with single crystals that have three distinct regions, from negative to positive: slope I, a saturation region II, and a slope marked III (Figure 3b). As previously described by the Parkinson group,20 region I corresponds to smaller band bending and lesser penetration depth, therefore, this is the region used for the Efb. The MS equation, eq. (1) was fitted to region I and extrapolating to the potential axis. Region III arises due to larger band bending and a space charge region that extends deeper into the semiconductor, thus, it yields the best estimate of the semiconductor donor density, ND. The slope of region III was used to solve for ND with eq. (1), the MS equation:33-34 1 2 = 2 2 CSC qA N Dεε 0

 k BT   E − E fb − q   

(1)

8


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where ε the dielectric constant of ZnO (ε = 9),35 ε0 is the permittivity of free space, q is the fundamental charge and ND is the carrier density. The values obtained are given in Table 1 with the error estimated for the extrapolation to y = 0 with the usual method (see SI, section XI).36 EC for single crystals can be calculated34 from equation (2) where NC is the density of states in the conduction band and taken to be24 NC = 3.5 × 1018 cm−3: − E C = E fb +

k BT  N D   ln q N C  

(2)

Figure 3. Mott-Schottky plots for single crystal electrodes terminated on the (1 0 −1 0) plane in 0.1 M TBAP. (a) Full potential scan and the CV in the same solution (note that the right hand side axis is 1/C2). (b) detail of the curve in (a) showing the three regions for 1/C2. (c) comparison of (black) TBAP solution without mediator, (blue) 1 mM Fc and (red) 2 mM TBAI. (d) Plot in 1 mM BQ showing the difference between (, red) a scan in the anodic direction and (⋅, blue) a scan in the cathodic direction.

9



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− Table 1. Results of the MS analysis with ZnO single crystal (1 0 1 0) Flat Band Potential

CB Edge

Efb V vs NHE

EC, V vs NHE

0.1 M TBAP

−0.194 ± 0.005(a)

−0.24

2 mM TBAI + 0.1 M TBAP

−0.061 ± 0.006(a)

−0.11

1mM Ferrocene + 0.1 M TBAP

−0.234 ± 0.005(a)

−0.28

Average

−0.163 ± 0.003(b)

−0.29

Electrolyte in CH3CN

±0.09

sn−1 1 mM Benzoquinone + TBAP(c)

−1.02 ± 0.03

−1.1

(a) estimated based on the x-intercept error in the MS plot in the cathodic scan in Figure 3 (SI, section XI).36 (b) Based on the error propagation of the measurements in three electrolytes. (c) Note that measurements for this couple are complicated by adsorption.

Figure 3 displays the MS results for the redox mediators used: Figure 3c shows that the MS results are consistent for (1) the 0.1 M TBAP solution without redox mediators, (2) the solutions with iodide and (3) the solution with ferrocene; these three Efb values are all within 90 mV. Figure 3d shows a different response for BQ that is consistent with the strong adsorption of the reduction product, BQ•−. The MS plot does not have a saturation region (II), and shows peaks that are strongly dependent on the direction of the scan. Thus, the Efb for this couple is taken with the stipulation that is probably shifted by the adsorption of the BQ0/-1 species. The results of Efb and EC are given in Table 2. The values of ND obtained were 5.5 – 5.8 × 1017 cm−3 consistent with previous reports.21 The EC in the electrochemical scale, i.e., in volts vs. NHE, can be converted to the absolute scale where vacuum = 0 eV using equation (3): − EC , vac = qEC + 4.5eV

(3)

10


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Taking the average value of EC for the single crystal is –0.3 ± 0.1 V vs NHE (but note that it depends on NC and ND), we obtain EC,vac = −4.2 ± 0.1, consistent with the levels obtained in vacuum of −4.4 to −4.1 eV.37-39

Cyclic Voltammetry in 0.1 M TBAP, no redox mediator. To study the position of EC for NWs cyclic voltammetry of heat treated NWs were performed in the dark and in the absence of mediators in 0.1 M TBAP (Figure 4a). This CV is consistent with the data for single crystal electrodes (Figure 4b); note the position of Efb measured by Mott-Schottky experiments. The CV for the single crystal shows background current smaller than the Pt control. There is no current at potentials positive of Efb and as the potential is scanned in the negative direction the background current starts to increase. Note that the NWs show a similar CV, with a larger background due to the higher ZnO surface available and to the presence of surface states. The broad features in the CV of NWs are due to surface-confined processes: they both increase linearly with scan rate (Figure S2). We also note that there is no significant difference from the CV of the NWs as prepared that have higher OH content on the surface (Figure S3). Figure 4c shows the NW current at E in the range of -0.5 V to 0 V that is assigned to states that lie below the CB and that are not present on the single crystal electrode (Figure 4b). We propose that they contribute to recombination and shift Eon. The scan rate of the CVs is 0.1 V/s, making the time scale on the order of the carriers detected around 0 V of the order of seconds, significantly higher than the time scale of pico- to nanoseconds reported for optical measurements of surface states around EC for annealed NWs in aqueous solutions as a function of potential.26 We attempted to characterize the NWs with the electrochemistry of I−/I3− and Fc/Fc+ redox couples but it is not possible to distinguish between the contribution of the ZnO NWs and that of FTO support for the redox processes studied in this work; however, 11



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photoelectrochemical studies were possible because of the negligible photocurrent of the FTO substrate. For BQ it was possible to distinguish against the FTO background and the CVs and photoelectrochemistry are discussed below.

Figure 4. CVs of ZnO in 0.1 M TBAP without additional redox mediators. The CVs were performed in the dark (a) Heat treated NWs (b) Single crystal electrode (↑) shows the Efb measured by Mott-Schottky plots. (c) detail from −1.5 to 1.8 V for NWs. (d) Control experiment for FTO support. (e) CV of a Pt electrode. Experiments where run starting on 0 V; the arrows indicate the direction of the initial segments. ν = 0.1 V/s

Photoelectrochemistry

in

0.1

M

TBAP,

no

redox

mediator.

We

performed

photoelectrochemistry experiments under chopped illumination during potential scans to determine the photocurrent onset, Eon, for the SC/electrolyte system (Figure 5).

For the

nanostructures Eon is significantly shifted to positive potentials due to recombination losses. To 12


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be fair, we determined the Eon for the main rise in photocurrent using the Butler method (described in SI, section IV).40 Figure 5 shows the photoelectrochemical measurements of ZnO NWs, ZnO single crystals and ZnO NPs ≈30 nm diam in 0.1 M TBAP in CH3CN under illumination by chopping the light; the data for pellets is shown in Figure S4. A larger iph was observed for NWs with respect to NPs films, single crystal and pellets. The larger iph is likely the combined result of larger surface area by the NWs and higher e- mobility in the wires due to their single crystalline nature, in contrast to the particulate nature of the NPs. Interestingly, the Eon for NWs is more positive than the ZnO NPs (Table 2). For comparison, the same procedure was done to the samples after adding iodide and the results are shown in Table 2.

13



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Figure 5. Linear sweep experiments under chopped illumination (50 mHz) on ZnO electrodes a) NWs 2.5 µm long and diam ≈150 nm b) single crystal electrode (c) NP d ≈30 nm, film thickness 0.4 µm,. The red curves in a) and c) show the chopping on blank FTO plate (heat treated). Potential scans were started at −0.5 V and ran in the anodic direction, ν = 1 mV/s. No redox mediator was added. 14


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Table 2: Photocurrent onset potentials obtained for different ZnO nanomaterials and single crystal in 0.1 M TBAP in CH3CN solution without iodide, and with 1.94 mM TBAI. a) The values shown are the average of three different electrodes; the error bars are ± σn-1

ZnO Materials

Eon vs NHEa)

Notes

Without Iodide 1.94 mM TBAI ZnO NPs (30 nm Film thickness 0.4 ± 0.1 µm diam)

0.86 ± 0.08

0.18 ± 0.01

ZnO NWs

1.4 ± 0.1

0.29 ± 0.01

Diam = 150 - 200 nm Film thickness 2.2 µm, as prepared

ZnO NWs

Heat treated. Length: 1 – 2 µm

1.06 ± 0.05

n/ab)

ZnO SC

No heat treatment (0.5 mm thick)

1.11 ± 0.01(c)

-0.084 ± 0.003

b) n/a: not available c) based on the error for the extrapolation to the y-axis

The results in Table 2 indicate that the photocurrent onset is positive with respect to I−/I3− couple for the three types of ZnO materials studied.

These results indicate that Eon is

significantly different from EC: several reports on DSSC based on ZnO and the I−/I3− electrolyte, including NWs.8-10,41 Because the DSSCs for ZnO NWs and NPs work with I−/I3− the EC of the ZnO nanostructures and for the pellet should be more negative than the I−/I3− formal potential, while our measurements in CH3CN with 0.1 M TBAP show the photocurrent onset potential to be positive with respect to the I−/I3− redox couple due to recombination losses. Interestingly, the NWs showed even more positive onset (at least +0.5 V) than NPs, single crystals (Figure 5b) and 15



pellets. We propose that these large shifts on the main photocurrent onset, Eon on the different ZnO materials used are due to recombination on surface states that causes the Eon to shift to a positive potential with respect to Efb and EC. Some of these surface states are of a hydroxide nature: experiments on the NWs as prepared, without heat treating show the most positive Eon, while heat treating the NWs (400 °C, 30 min) makes the Eon shift negatively by 0.4 V. The IR shows the characteristic OH stretching that is significantly reduced after heat treatment (Figure 6). Therefore, we propose that OH on the NWs surface increases the recombination, as evident from the larger shifts on photocurrent onset. Note that the OH-bound states only account for some of the differences observed between NWs and the other ZnO electrodes of NP films and a pressed pellets, therefore, further experiments were performed that indicate the presence of additional surface states.

0.014

0.0028

Absorbance

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0.007 3000

0.0014

0.0000 3200

3400

3300

3600

Wavenumber (cm -1)

Figure 6. FTIR spectra of ZnO NWs scraped from FTO substrate showing hydroxide peak at 3360 cm−1. The main graph shows the subtracted IR spectra and the inset shows the individual data sets: red curve for as prepared and black curve for heat treated NWs.

Photoelectrochemistry

of

Iodide.

We

studied

the

effect

of

I−

ions

on

the

photoelectrochemistry and Eon by adding 1.9 mM tetrabutylammonium iodide (TBAI) to the 16


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0.1 M TBAP electrolyte. Figure 7 shows the photoelectrochemistry of NW (Figure 7a) and single crystals (Figure 7b). The calculated Eon has been summarized in Table 2 and includes the values for NWs and NPs. Figures S6 and S7 shows the photoelectrochemical experiments and the calculation of Eon in the presence of I− for ZnO NWs (Figure S6a and b) and NPs 30 nm diameter (Figure S7c and d). For NWs, the main photocurrent envelop is now around −0.4 V vs I−/I3−, and Eon is significantly shifted to negative potentials for NWs as well as NPs. Close inspection of the graph reveals a smaller photocurrent at E < −0.1 V. Comparing the region at potentials negative of the reference for NW (Figure 7c) and the single crystal (Figure 7a) it is apparent that the photocurrent onset for NWs is more positive than for the single crystal electrode. For the single crystal, the onset is consistent with the Efb measured with Mott-Schottky plots. Thus, we assigned the shift on the onset potential of NWs to surface recombination losses. Another feature that supports recombination is the rapid decay of the photocurrent after illumination is turned on, apparent in Figure 7.

17



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Figure 7 Photoelectrochemistry of 2 mM TBAI + 0.1 M TBAP for (a) NWs and (b) Single crystals. (c) and (d) show the −0.65 to 0 V region vs I−/I3− reference; all other conditions as on Fig. 4. We studied the effect of iodide ions on the ZnO NWs by imaging a NW electrode with scanning electron microscopy (SEM). We compared images of NWs that were subjected to photoelectrochemical tests in solutions of 0.1 M TBAP without I− (Figure 8a) and with 1.9 mM I− (Figure 8b). Clearly, the experiments show etching in the absence of iodide (Figure 8a) while the NWs exposed to I− solutions (Figure 8b) are indistinguishable from the NWs prior to exposure (Figure 2a). These experiments indicate that the current observed in the absence of iodide is due to the photooxidation of ZnO that is caused by photogenerated h+, and if I− is 18


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present in solution, the h+ are removed protecting the NWs. These results imply that the photocurrent onset in the absence of iodide is shifted from EC to provide enough driving force for h+ to oxidize O2- in the ZnO lattice.

Figure 8. SEM images of ZnO NWs after photoelectrochemical experiments with 0.1 M TBAP, (a) in absence of TBAI (b) in presence of 1.9 mM TBAI; the scale bar is 1 µm. The insets in both (a) and (b) show high resolution images respectively; inset scale bar is 250 nm.

Photoelectrochemistry of Ferrocene. We studied the photoelectrochemistry of the ferrocene (Fc)/ferrocenium (Fc+) couple to assess the position of the conduction band edge. The photocurrent onset of these NWs as obtained in 1 mM Fc is shown in the SI (Figure S8). The experimental data indicates the photocurrent onset for photooxidation of Fc for NWs near 0 V vs I-/I3- while Fc/Fc+ E0’ = 0.2 V vs I-/I3- (10 mM) as measured on Pt (not shown). At around 0.2 V a sigmoidal background anodic current shifts the dark current, indicating that this anodic background is due to the oxidation of Fc on FTO exposed due to defects on the ZnO coverage. This also indicates that the CB edge of ZnO is more negative of the redox potential of Fc/Fc+ because the ZnO NWs photooxidize Fc while there is no evidence that the NWs oxidize Fc in the dark: the photocurrent starts before the onset of the anodic background. Figure S8 also shows the data obtained with a single crystal (Figure S8c and d). Note that if the redox potential of the 19



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couple is negative of the Efb, the oxidation in the dark would be appreciable because the surface of the semiconductor becomes metal-like at potentials negative of Efb. Overall, the behavior of the semiconductor is consistent with the Fc/Fc+ potential being positive of Efb.23 In general, the NW photocurrent follows the same trend as the single crystal, but the photocurrent onset in NW is shifted towards more positive potentials. Also, in the details shown in Figures S8(b) and (d) it is apparent that the photocurrent decays quickly after illumination is turned on; this is consistent with surface carrier recombination. Although other processes are possible, such as filling of surface states with holes, we propose that the evidence described below with other redox couples support the assignment to carrier recombination (for example, there is no evidence of additional redox processes at potentials negative of 1 V vs the reference). Because there is no net oxidation of Fc in the dark which places the Ec value negative of the formal potential for Fc/Fc+ of 0.2 V vs the reference (10 mM I−/I3−). Both I−/I3− and Fc/Fc+ have significant background current on FTO (e.g., the background on Figure S8c). The BQ0/-1 on FTO on the control CV experiments shows much lower currents in the potential range of interest. Therefore we discuss the CVs and photoelectrochemistry with BQ.

Electrochemistry of p-Benzoquinone. In order to further investigate the position of conduction band edge and the effect of recombination on ZnO NWs we performed CV experiments in 1 mM p-benzoquinone (BQ0/−, E0’ = -0.54 V vs SCE,23 E0’ = -0.69 V vs I−/I3−) + 0.1 M TBAP. The CVs of the NWs in the dark show two distinct reduction peaks, A1 and A2. A1 is positive with respect to the reduction potential of BQ0/− and it is assigned to adsorption on the surface of ZnO; peak A2 is assigned to the main reduction of BQ/BQ− on the surface of NWs; A2 is more negative than the reduction of BQ on Pt (inset in Figure 9a). This is consistent with the EC of ZnO NWs around the formal potential for BQ/BQ•− reduction. The CV of BQ on 20


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single crystal electrodes only shows one cathodic peak with E1/2 ≈ −1.0 V, this CV is in line with the expected behavior of an n-type semiconductor in the dark: the BQ is reduced at potentials negative of Ec with no reverse for the oxidation of BQ•−. However, the NWs do show a reverse wave oxidation for peak A2; this oxidation is only seen on the reverse scan and therefore, it is assigned to surface states near the conduction band. The behavior for BQ under illumination is much more complicated for both the NWs (Figure 9d) and the single crystal electrode (Figure 9e). This behavior is assigned to the strong adsorption of the reduced form of p-benzoquinone. The photoelectrochemistry on both NWs and single crystals is shown in the SI (Figure S9) and displays a small photocurrent envelope at ca. −0.9 V. Based on the Mott-Schottky experiments on single crystal electrodes that display behavior consistent with BQ•− adsorption we assign these photocurrents to the adsorption of BQ•− on the ZnO surface.

Figure 9. CVs of para-benzoquinone (BQ) for NWs in (a) and (d) and for single crystal electrodes in (b) and (e, colored curves). (a) CV of FTO/NWs in the dark; the inset shows the CV of BQ/BQ− on Pt. (b) CV of ZnO single crystal in the dark: (, blue) corresponds to the BQ 21



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solution and (, red) to the 0.1 M TBAP blank. (c) Voltammogram for FTO in blank. (d) CV under illumination for FTO/NWs. (e) CV under illumination for single crystal electrode. Note: (a) and (b) compare CVs in the dark while (d) and (e) under illumination for the two kind of electrodes; ν = 100 mV/s, and 1 mM BQ in 0.1 M TBAP; blank solution is 0.1 M TBAP.

Role of recombination vs. adsorption in shifting Eon. Adding iodide and ferrocene shift the NW photocurrent onset significantly to more negative potentials (Figure 7, Figure S8, and the data in Table 2). The shift in Eon is due to the differences in oxidation kinetics of iodide ions, ferrocene and benzoquinone versus ZnO oxidation and recombination, as we describe below; here address the effect of adsorption. These experiments rule out the possibility that tetrabutylammonium (TBA+) adsorption shifts Eon to potentials positive of Ec. Cation adsorption could shift Efb toward positive values,42-43 for example, Nakabayshi et al. reported the positive shift in Efb due to the adsorption of positively charged photoionized dye molecules on the electrode surface.44 However, the concentration of TBA+ ions of 0.1 M is 100 times that of Fc (1 mM) and 50 times that of iodide (2 mM). Thus, it is unlikely that a strongly adsorbed TBA+ would be displaced from the ZnO surface by these couples. In particular Fc being neutral is not expected to displace TBA+ from the ZnO surface. Also, the experiments with single crystals do not show evidence of iodide or ferrocene. Therefore, we propose that adsorption of TBA+ ions on the ZnO surface is not responsible in shifting the Efb, rather, we propose that the effect of recombination is responsible on shifting the onset potential to positive values. For BQ there is a significant shift on the photoelectrochemistry (Figure S9) as well for the MS data (Figure 3) indicating that the strong adsorption of the reduced form (BQ•−) shifts the Efb.

22


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Conduction Band Edge for NWs and Single Crystals. To summarize the electrochemical experiments described above, EC from MS analysis on ZnO single crystal is EC = –0.3 ± 0.1 V vs NHE (Table 1). In considering the CV experiments in NWs and their comparison to the single crystal CV in TBAP without mediator, the CVs are consistent with the NW having the same EC value. We note that there are large differences in the photocurrent onset, Eon in Table 2 with this value. There are also significant differences with the BQ experiments that are due to adsorption of BQ and to the presence of surface states around EC.

Nanowire photooxidation. The results discussed above, CVs and photoelectrochemical experiments with and without redox mediators, indicate that for the NWs EC is at a negative potential with respect to the photocurrent onset potential of the main photocurrent envelope, i.e., that the Eon shifts to positive values due to recombination. The EC ≈ Efb should then be approximately −0.65 V vs I−/I3− or –0.3 ± 0.1 V vs NHE. In the absence of redox mediators, i.e., 0.1 M TBAP in CH3CN, the relatively large photocurrent observed is due to the photooxidation of the ZnO. Semiconductors in electrochemical systems are usually kinetically stabilized rather than thermodynamically,45-47 and ZnO is not stable in aqueous solutions45 but it is stable in nonaqueous systems like MeCN.23 As mentioned above, evidence for photooxidation of ZnO comes from the SEM of NW films (Figure 8) before and after photoelectrochemistry experiments in solutions of 0.1 M TBAP: the SEMs show that the NWs are etched after the photoelectrochemistry experiments without iodide (Figure 8a). In contrast, a film treated under the same conditions with I− does not show the same etching effect (Figure 8b). Therefore, the photoanodic current in the absence of redox mediators is likely due to a decomposition reaction: 2ZnO + 2h+ 2 Zn2+MeCN + O2,MeCN

(4)

23



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where the CH3CN subscript indicates that the species are solvated in CH3CN. To the best of our knowledge, the solvation energies are not known, so it is not trivial to estimate the potential of this reaction, Vd, based on thermodynamic quantities. However, the well-known stability of ZnO in nonaqueous systems indicates that this potential is positive of valence band edge, EV. Note that there is no significant anodic current in the dark in the photoelectrochemical experiments in Figure 5 and in the NWs voltammograms there is no significant anodic peak at potentials positive of the peak assigned to EC, all the way to 1.8 V vs 10 mM I−/I3−; this indicates that there is no corrosion under reverse bias in the dark (Figure 4); however, there is an anodic current under illumination in all the experiments. Therefore, the observed photocurrent in the absence of mediator should be due to the band bending, ∆ϕ caused by the applied potential that drives the photocorrosion of ZnO under reverse bias that increases the oxidation power of h+. ∆ϕ is related to the depletion width, W by:30,33

 2εε 0  W =  ∆ϕ   qN D 

1/ 2

(5)

Where all symbols have their usual meaning as in eq (1). Reports on ND of ZnO single crystals are in the order of 6 × 1016 and 5 × 1017 cm−3 (ref 21), while for NWs (subjected to similar heat treated conditions used here) 5×1017 cm–3 have been reported.30 Therefore, a depletion width of 60 < W < 200 nm is expected for 6 × 1016 < ND < 5 × 1017 cm−3 and a band bending of ∆ϕ ≤ 2 V. Our most positive photocurrent onset in Table 2 is 1.4 V and EC = −0.3 V vs NHE so in our experiments photocurrents become apparent at ∆ϕ < 2 V. Because of the dimensions of the wires, diameter 100 – 200 nm and length of 1- 2 µm, the wires are capable of sustaining this depletion width along the main c axis. The photocurrent onset of the anodic decomposition of ZnO is observed at 1 V for the heat treated NWs. For Eon, ≈ 1 V vs NHE and taking for the NWs 24


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Efb ≈ EC = –0.3 ± 0.1 V vs NHE, ∆ϕ ≈ 0.7 V, and in turn, this will place the onset of anodic dissolution of ZnO at around 0.7 V positive of the EV of ZnO, i.e., Vd ≈ +3.7 V vs NHE given the band gap (Eg = 3.3 eV). Figure 10a shows this value and summarizes our findings.

Figure 10. (a) Energy diagram of ZnO NWs showing the proposed distribution of surface states, the dissolution potential, Vd and the redox potentials of the redox couples are given in the diagram, including the oxidation of MeCN at +2.4 V vs NHE. (b) Schematic representation of a ZnO NW.

Surface Recombination. For efficient charge separation to occur in a NW, the minority charge carrier must be sufficiently long-lived to overcome recombination within the dimensions of the NW. Lewis and co-workers have proposed that these conditions can be summarized by the relationship between NW radius, r and length, l, and the surface recombination rate, S, so that S < Dminr/l 2, ref

30

, where Dmin is the diffusion coefficient of the minority carrier. In our

experiments, recombination is prevalent with and without redox couples at low ∆ϕ: the minority carriers do not have sufficient lifetime to be able to diffuse across the NWs and be collected unless substantial bias and ∆φ is applied. Therefore, we can estimate the lower limit of the recombination rate at the surface states by rearranging the conditions for S given the dimensions of the NWs: 25



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S≥

Dmin r l2

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(6)

This relationship strictly applies for the high level injection, i.e., when the carriers have to diffuse along the length of the NW because the depletion width is larger than the radius, or W > r which given the variability in ND, this condition is fulfilled at ∆φ > 2 V (equation 5). We have performed experiments at these large biases without appreciably increasing the photocurrent (e.g., Figure S6). Therefore, equation 6 can be used to estimate a lower limit for S for the NWs under a depleted core30 (high level injection) because the carrier collection efficiency did not change at large biases. For ZnO NWs, the parameters on the right-hand side of eq (6) have been reported for NWs prepared with similar methods.30 Dmin = kTµmin/q, where k is Boltzmann’s constant and µmin is the mobility of the minority carrier, i.e., h+ for ZnO NWs, µmin = 10 cm2 V−1 s−1.30 Taking Dmin = 2.6×10−1 cm2/s, r = 100 nm and l = 2.2 µm so that for the NWs in this work S ≥ 53 cm/s from eq (6). This is a significantly large value of surface recombination48 and explains why recombination predominates at low values of ∆ϕ. It is consistent with the photocurrent onset potential being more positive than EC for NWs with all the redox couples used in this work. The redox couples include (a) the facile outer sphere, one electron reaction Fc/Fc+ (which is intrinsically fast and presents considerable currents on FTO),49 (b) the BQ/BQ−, and (c) the more complicated inner sphere, asymmetric kinetic redox system of I−/I3− that provides a more effective way to overcome recombination.50

Because wires have a more

positive shift for the onset potential than single crystals and NP films, then the S should be larger on the wires, despite their single crystal nature that confers them significantly larger mobility with respect to NP films.

26


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We propose that the recombination centers are the result of surface states that arise from the termination of the planes on the ZnO NW surface and from the OH that terminate the ZnO structure. The states that are responsible for recombination are not observed on the photocurrent experiments of all the redox couples. The EC and approximately Efb is assigned to –0.3 ± 0.1 V vs NHE as discussed above. We note that recently, Liang et al.20 reported much more positive values for single crystals (0.28 V vs NHE) although the heat treatment that these authors used at 1000 °C cannot be reproduced with NW on FTO. This is positive of the previously reported single crystal data (Efb negative of -0.5 V vs NHE) with a different terminal plane (0 0 0 1).23 Under quantum confinement, EC of QDs shifts to electrochemical potentials negative of −0.63 V vs NHE for 8.6 nm diam. particles,25 consistent with our proposed NWs EC values. For NWs, the onset potential is found to be more positive than for NPs, and pellets. The photocurrent onset was also found to be more positive for NWs than for single crystals with and without iodide (Table − 2). This is attributed to the surface states that form due to the terminated (1 0 1 0) planes of

hexagonal wurtzite structure of NWs that comprise most of the expose surface (Figure 10b), so these planes formed of mixed Zn and O atoms and their surface energies should play a key role on determining the electrochemical properties of the NWs. The EC for both NWs and single crystals is the same based on the CVs in 0.1 M TBAP. The CVs for the BQ couple show the most striking differences (Figure 9a), where the reduction of BQ shows a reversible wave consistent with the oxidation of the reduced BQ•−, this wave is not present on the single crystal (Figure 9b). These differences are assigned to surface states on the NWs that arise because the − − intersection of the (1 0 1 0) planes. Marana et al.51 point out that the (1 0 1 0) planes should be

unstable due to the dangling bonds.52 We propose that the NWs do not significantly change their 27



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− EC or Efb with respect to the bulk single crystal, rather, the (1 0 1 0) planes and their re-

construction result in surface states. Experimental elastic low-energy-electron diffraction (ELEED) and computational work indicating that surface rearrangements include the top layers of Zn and O being displaced downwards.53. On the NW surface, it is also possible that along the − intercept of two (1 0 1 0) planes the surface may reconstruct to give rise to surface states that are

not seen on single crystals. Our results indicate that the surface states on NWs would span ≈ 0.7 eV below the CB and shift the photocurrent onset potential; also note that there is evidence of surface states on the voltammograms (Figure 4) and of states in the UV-Vis of the NWs films/FTO (Figure S11) and with the UV-Vis of a suspension of NWs in CH3CN (Figure S12); additional evidence comes from the widely reported luminescence of NWs with peaks assignment still being controversial (e.g., refs

3,54

). Also, the NWs can be terminated on OH,

increasing the recombination rates as evident in the change of the photocurrent onset: the as prepared NWs terminated with OH− have Eon values shifted 0.4 V more positive than the heat treated NW (Table 2). These recombination centers are very effective at competing with fast, asymmetric redox process such as the photooxidation of I−, and a minimum for the recombination rate, S, can be estimated based on the lifetime requirement of the carriers: S ≥ 53 cm/s. In addition, we can propose limits for the electrochemical stability of ZnO, by estimating the Vd position, which lies positive with respect to EV and of the CH3CN oxidation potential.55 All these values are illustrated in Figure 10 that summarizes the findings of our study.

Conclusion

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We propose the position of the conduction band edge of NWs to be –0.3 ± 0.1 V vs NHE based on Mott-Schottky plots of single crystal electrodes and on CVs of NW and single crystal electrodes. This is positive with respect to the QDs with EC negative of −0.63 V vs NHE for NPs smaller than 8.6 nm diameter.25 The photoelectrochemical onset potential was found to be more positive than EC due to surface recombination losses that are estimated to be S ≥ 53 cm/s for ZnO NWs. This recombination rate successfully competes with the rate of h+ reaction with redox couples of Fc, I− and BQ. Formation of a hydroxide layer increases the surface recombination rate in non-aqueous electrolytes. Recombination is attributed to surface states that form because − most of the surface area of NWs consists of (1 0 1 0) planes (Figure 10b) and there is likely

surface reconstruction to minimize the energy of the mixed Zn and O surface.53 Our results are in line with NW device behavior that show a difference of ca. 0.1 V in open circuit potential when compared with devices made of single crystals.30 The reduced form of BQ is found to shift the EC to negative potentials. Future work will include studies of NW and single crystal surface reconstruction with high resolution scanning probe microscopy.20

Acknowledgments This project was supported by NSF CAREER Award No. CHE-1255387 and by UNR startup funds for MAA. Supporting Information Available Experimental details and additional experimental data are available. This information is available free of charge via the Internet at http://pubs.acs.org.

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(a) energy diagram of ZnO NWs showing the proposed distribution of surface states, the dissolution potential, Vd; the redox potentials of the redox couples are given in the diagram, including the oxidation of MeCN at +2.4 V vs NHE. (b) schematic representation of a ZnO NW 95x37mm (300 x 300 DPI)


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51x44mm (300 x 300 DPI)


he Journal of Physical Page 40 Chemistr of 40

1 2 3 4 Paragon Plus Environment ACS 5 6 7

Control of Carrier Recombination on ZnO Nanowires

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