Photoelectrochemical Investigation of the Mechanism of Enhancement

Article pubs.acs.org/JPCC

Photoelectrochemical Investigation of the Mechanism of Enhancement of Water Oxidation at the Hematite Nanorod Array Modified with “NiBi” Lara Halaoui*,†,‡,§ †

Department of Chemistry, American University of Beirut, Beirut 110236, Lebanon Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, United States

‡

S Supporting Information *

ABSTRACT: Modifying the surface of a hematite photoanode with “NiBi” or “CoPi” oxygen evolution catalysts has been reported to reduce the bias required for photooxidation of water, but the mechanism remains a subject of discussion. In this work, the effect of photochemically modifying an array of hematite nanorods in Ni2+ solution in potassium borate (or NiBi) on water oxidation was studied using cyclic voltammetry in the dark and in the light and with photocurrent transient measurements. A prominent reduction of a photooxidized species is reported here on the reverse scans of cyclic voltammograms under illumination near the photocurrent onset, coinciding with the cathodic shift in water oxidation potential, showing transfer of the hole in a longlived oxidized state at NiBi-hematite. The reduction peak was still observed at slow scan rate indicating a stable photooxidized state with slow back electron reaction. A reduction was similarly detected albeit with lower magnitude in the dark when the modified electrode was biased to high anodic potential into the water oxidation region. Photocurrent transients revealed slow decay with a median lifetime of ca. 1−3 s at low bias at NiBi-hematite and a slower decay relative to the bare electrode biased 0.6 V more positive. The results are explained by proposing that photogenerated hole transfer occurs to a “state” at Ni-oxide-modified hematite, forming a photooxidized species where the photohole is sufficiently long lived, with reduced recombination rate allowing water oxidation to occur at low bias. Electrochemical evidence points to this being the same oxidized state formed in the dark at water oxidation potentials at NiBi-hematite, but the chemical identity of the photooxidized state involved in water oxidation is yet to be determined.

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INTRODUCTION

Electron−hole (e−h) recombination is fast at unbiased hematite, with rates reported as fast as the picosecond time scale,12−14 and competes with slower hole transfer to water (∼3 s).15−17 Despite a significant built-in overpotentialthe difference between the valence band edge and the 4-e water oxidation potentiala bias of 0.3−0.4 V vs the flat band potential is needed for water oxidation at hematite. The anodic bias serves to slow e−h recombination by enhancing band bending (in bulk electrodes) or depleting the density of electrons (in nanostructured electrodes). Because the conduction band of hematite is lower than the proton reduction potential, a bias is required to drive H2 evolution at a counterelectrode and can be provided by a tandem cell.15,18,19 The oxygen evolution reaction (OER) is a kinetically demanding reaction since it involves the transfer of four electrons and four protons to produce O2 from two water molecules. In nature, this reaction is affected during photosyn-

The efficient splitting of water using solar energy at a semiconductor to produce hydrogen fuel has been a holy grail in chemistry researched for decades since a first report by Fujishima and Honda in 1972 that showed this photoelectrolysis at a TiO2 photoanode and a Pt counterelectrode.1,2 Among the many metal oxides investigated for the photoanode is hematite, α-Fe2O3, an n-type semiconductor with a band gap of 2.1−2.2 eV.3−6 Hematite is stable, earth-abundant, and environmentally benign and has a band gap that allows absorption of a good portion of the solar spectrum and a suitably aligned valence band edge for water oxidation.7−9 However, the material is plagued by a short minority-carrier diffusion length of only 2−4 nm7 or 20 nm8 and reportedly by sluggish kinetics for hole transfer to water.8,9 In addition, its indirect band gap causes low absorbance close to the absorption edge, with a photon penetration length ∼0.1−1 μm at 500−600 nm,9−11 significantly longer than the hole diffusion length. Therefore, holes generated far from the surface will recombine before reaching the semiconductor/liquid interface. © XXXX American Chemical Society

Received: May 2, 2016 Revised: September 7, 2016

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DOI: 10.1021/acs.jpcc.6b04430 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. SEM images of a hematite NR array (A) and following its photochemical modification in a solution of Ni2+ in potassium borate (B).

described by Lin and Boettcher as an “adaptive” junction with varying Schottky barrier height as the ion-permeable electrocatalyst is oxidized.42 Their study indicated hole transfer from TiO2 to NiOx moving the latter Fermi level until suitable to drive water oxidation, consistent with hole accumulation in CoPi on hematite.42 A different hypothesis emerged from studies by Durrant and co-workers on the dynamics of photogenerated holes using transient absorption (TA) measurements at hematite modified with Co-oxide: that the redox layer enhances electron depletion or band bending in hematite, lowering charge recombination with no evidence of charge transfer to the catalyst, the long-lived hole remaining a hematite hole.33 A recent report by Gamelin and co-workers proposed that CoPi at low coverageby accepting the photoholedecreases hole accumulation at hematite, reducing Fermi level pinning and enhancing depletion; the authors report slower water oxidation at CoPi-hematite than at hematite.43 Understanding the mechanism by which these redox layers enhance the photoanode properties is essential for designing cocatalysts or surface modification to facilitate water splitting. In this study, hematite nanorod arrays44 were photochemically modified in Ni2+ in potassium borate, and the effect of modification at different surface coverage was interrogated using cyclic voltammetry in the light and in the dark and using transient photocurrent measurements. A major cathodic discharge peak is reported in reverse negative scans in cyclic voltammograms under illumination at NiBi-hematite, near the photocurrent onset, coinciding with the cathodic shift in the bias for water oxidation caused by modification. A cathodic peak was observed in the dark at similar potential when Nioxide-hematite was biased into the oxygen evolution potentials, and a cathodic peak was observed with significantly lower magnitude before modification. Photocurrent transients revealed slow photocurrent decay at low bias after modification with a lifetime in the second time scale, of ca. 1.5−3.4 s at −0.1 V vs Ag/AgCl, and slower than at the bare electrode biased 0.6 V more positive. The results can be explained by proposing that hole transfer occurs to a Ni-oxide-hematite “state” forming a stable photooxidized species that exhibits less facile electron back reaction, making the photooxidized state sufficiently longlived at low bias, thus lowering the bias required for water photooxidation. Electrochemical evidence points to this being the same oxidized state formed in the dark at water oxidation potentials at NiBi-hematite, but its chemical identity still needs to be determined.

thesis by an oxygen evolving complex (OEC) consisting of a Mn-based oxide (Mn4CaO4).20 Bard pointed out in a perspective on the subject that in the absence of multielectron catalysts water oxidation at semiconductor electrodes would occur by a series of single-hole transfers to water species, which are thermodynamically more demanding than the four-electron process.21 Durrant and co-workers showed that the lifetime of the long-lived hole responsible for water oxidation at hematite is independent of the hole concentration, an indication of sequential single-hole oxidation steps, at the conditions used.17 This prompts coupling semiconductors, even those having large built-in overpotential with multielectron catalysts for water oxidation. High and sustained activity for oxygen evolution has been observed by Nocera and co-workers at electrochemically deposited amorphous Co- and Ni-based oxide films in the presence of phosphate and borate buffers.22−25 Deposition of Co- and Ni-oxo/hydroxo OER catalysts on nanostructured and bulk hematite reportedly lowered the bias required for photooxidation of water by 0.15−0.3 V.26−34 The mechanism however remains a subject of discussion, with electrochemical and spectroscopic observations at hematite modified with Cooxo/hydroxo in phosphate buffer (CoPi) leading to different interpretations.33−35 Modifying hematite with Ni-oxo/hydroxo films in borate (NiBi) was also reported to lower the bias required to drive water photooxidation, but the system has been less investigated than CoPi.32 The NiBi catalyst has a small Tafel slope of ∼30 mV/dec at low overpotential and significant catalytic activity for oxygen evolution in thin films.24,25 Several recent reports also prompt interest in various effects of hematite modification with Ni.36−39 Studies of the role that an OER catalyst such as CoPi plays in lowering the bias for water photooxidation at hematite have led to different hypotheses. Klahr et al. reported a photoelectrochemical and impedance study at thin hematite films modified with CoPi with varying thickness, proposing that hole storage occurs as Co(IV), which facilitates charge separation, followed by catalyzing water oxidation.34 A long-lived Co(IV)oxo intermediate and bimolecular water oxidation mechanism were invoked,34 instead of the Fe(IV)-oxo intermediate proposed at bare hematite.40,41 Water photooxidation via the catalyst was also proposed by Gamelin and co-workers at CoPi on mesostructured hematite;35 the authors proposed that water oxidation occurs via CoPi when the layer is thick or via either CoPi or directly via hematite depending on potential when the layer is thin.35 The TiO2/porous Ni-oxide contact has been B


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RESULTS AND DISCUSSION Hematite NR Arrays and Surface Modification. Hematite nanorod (NR) arrays were photochemically modified in 0.4 mM Ni2+/0.1 M potassium borate (KBi) aqueous solution by illuminating with white light, for different lengths of time to vary coverage. Figure 1 presents SEM images of a forest of hematite NRs before and after modification in Ni2+/KBi. The micrographs showed NRs of 113 ± 21 nm diameter (N = 100 rods from Figure 1A) and some spherically shaped nanostructures on the modified NRs that could be photodeposited Ni(oxide) nanoparticles. The absorbance spectra of the NR arrays are presented in Figure S1 following two annealing steps (to 550 °C, then to 800 °C). The second annealing step to 800 °C was necessary because minimal photocurrents were recorded after annealing to 550 °C but must be brief as it softens the glass. The improvement of electronic properties may be due to Sn diffusing and doping the films.45 EDX showed Ni on the modified NRs, as presented in Figure S2 for hematite modified by illumination in Ni2+/KBi for 45 min and 1 h 15 min to allow detection of Ni above background. Electrochemical Study in the Dark at Hematite NR Arrays Modified in Ni2+/KBi. Figure 2A shows cyclic voltammograms (CVs) of hematite in 1 M KBi (pH 9.2) before and after photochemical modification in Ni2+/KBi for t = 20 min, termed Ni−20 for reference. The potential sweep at bare hematite between 1.8 V and −0.4 V shows the absence of faradaic processes until ca. 1.0 V where oxygen evolution begins. At an n-type bulk semiconductor under extreme polarization positive of the flat band potential (Efb) significant band bending occurs, and redox processes can take place in the dark via the conduction band by electron tunneling through the space charge region (SCR), leading to dark electrochemistry at positive potentials. At nanowires under anodic bias the effect may be a combined band bending and depletion without band bending,17 depending on the width (W) of the space charge region46 in relation to the dimension at the applied potential. As an example, assuming 1017−1019 cm−3 carrier concentration at undoped hematite and a dielectric constant of 80,47 W for a 1 V Schottky barrier is ca. 30−300 nm47 or is calculated46 to be ca. 16−163 nm for a 0.3 V barrier, for instance, therefore either smaller or larger than the nanorod dimensions, of average width ca. 110 nm and length in the range of ca. 500 nm. The carrier density has been reported at ca. 5 × 1019 cm−3 at hematite nanowires annealed to 800 °C;45 a greater carrier density will cause a decrease of the width of the space charge region.46 The dark CV of Ni−20/hematite features an oxidation shoulder at ca. 0.90 V (A1) followed by oxygen evolution and a cathodic peak with Epeak at ca. 0.70 V (C1) and a broad reduction peak that appeared Gaussian with Epeak at 0 V (C2) on the reverse sweep. Modification of hematite also lowered the overpotential for oxygen evolution in the dark by 0.110−0.120 V. The A1/C1 redox peaks indicate quasi-reversible behavior and correspond to the reported Ni(OH)2/NiOOH redox transformation at Ni-oxide electrodes.22−25,48 The same A1/C1 peaks were observed at an electrochemically deposited NiBi on FTO (Figure S3) and are in a similar potential range as reported by Badiako et al. in borate pH 9.2with Ep,a at 1.05 or 1.025 V vs NHE (0.853−0.828 V vs Ag/AgCl) and Ep,c at 0.81 or 0.87 V vs NHE (0.613−0.673 V vs Ag/AgCl) depending on conditioning.49 Nocera and co-workers reported at oxygen evolution potential a Ni oxidation state of 3.16 or 3.6 before and after anodic conditioning, respectively.49 The peak

Figure 2. (A) Cyclic voltammograms at a hematite NR array before (a) and after (b) modification in Ni2+/KBi for 20 min (Ni20). Inset corresponds to the CV of the unmodified hematite NR array. (B) CVs at two different hematite NR arrays modified in Ni2+/KBi by illumination for 10 min (Ni−10) (film 1) scanned to 1.6 V positive limit and a second film (film 2) illuminated for 20 min (Ni−20, b) then an additional 10 min (Ni−30, c) scanned to 1.8 V positive limit. Electrolyte is 1 M KBi pH 9.2. Scan rate is 100 mV/s.

potentials of this transformation are known to be affected by the phase, α-Ni(OH)2 or β-Ni(OH)2, hence by aging, anodic bias, and inclusion of ions such as Fe.50 The cathodic peak C2 is not present in CVs of NiBi on FTO (Figure S3), and there was no other reduction in this potential range (Eo of Ni2+/Ni and Ni(OH)2/Ni are at −0.257 V and −0.72 V vs NHE,51 respectively). At bare hematite (inset of Figure 2A), a small reduction peak with more anodic Epeak at around 0.20 V and a significantly smaller current density (ipeak = 9.5 × 10−3 mA/ cm2) was observed compared to C2 at Ni-hematite (ipeak = 1.35 × 10−1 mA/cm2). Figure 2B shows CVs in the dark in 1 M KBi at a NR array modified in Ni2+/KBi by illumination for t = 10 min (a) and a second array illuminated for 20 min (b) and 30 min (c), to monitor the effect of increasing coverage on the electrochemistry. The A1 and C1 peak currents and charges increased with illumination time and can be used as a comparative measure of coverage of (electroactive) NiBi on different electrodes. The C2 peak magnitude also increased with illumination time in Ni2+/KBi, best seen in the CVs at the C


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The Journal of Physical Chemistry C same hematite film modified with Ni−20 and Ni−30 and scanned to the same positive limit of 1.8 V (cf. plots b and c in Figure 2B). To investigate the oxidation process at Ni-hematite that leads to the C2 cathodic peak, the dependence on the positive scan limit was studied. Figure 3 shows CVs at 100 mV/s at Ni−20/

Figure 4. CVs at a hematite NR array (a) and at the modified film Ni−10/hematite (b) under illumination at 100 mW/cm2 and a CV in the dark at Ni−10/hematite (c). The inset shows the photocurrent onsets. Initial scan direction is from negative to positive potentials. Electrolyte is 1 M KBi pH 9.2. Scan rate is 10 mV/s.

sustain photooxidation of water at Ni-hematite at the same rate as before modification was reduced by a maximum of 0.315 V in I−V curves at 10 mV/s. This cathodic shift is consistent with reports that photochemical or electrochemical deposition of Nioxide and Co-oxide on hematite lowered the bias for water photooxidation by 0.1−0.3 V.26−34 The photocurrent onset also shifted (inset) from ∼−0.008 V (0.74 V vs RHE) at hematite to ∼−0.14 V (0.60 vs RHE) at Ni-hematite in borate buffer pH 9.27. The photocurrent onset for water oxidation at the NRs is more negative than reported at other hematite films40 and is near Efb reported at thin hematite film at 0 sun (−0.22 V vs Ag/ AgCl or 0.756 V vs RHE at pH 13.2) but more negative than measured at 1 sun (0.906 V vs RHE).41 This could be caused by the nanorod architecture facilitating charge separation without decreasing absorption by decoupling the directions along which these processes take place. Increasing NiBi coverage by increasing time of illumination led to a more significant scan rate dependence of I−V plots under illumination. Figure 5 presents photocurrent−voltage curves at Ni−10/hematite (A) and Ni−25/hematite (B) at 10− 200 mV/s and at unmodified hematite at 10 and 100 mV/s in 1 M KBi. The anodic sweeps under illumination at Ni−10hematite show minimal scan rate dependence between 10 and 100 mV/s and a cathodic shift of 0.040 V from 10 to 200 mV/s. I−V curves at Ni−25/hematite exhibited a greater scan rate dependence, and the maximum cathodic shift was ca. 0.280 V at 100 mV/s compared to 0.214 V at 10 mV/s, while the photocurrent onset shifted by ca. −0.190 or −0.216 V, respectively. A significant scan rate dependence was also measured at Ni−30/NR arrays (Figure S7). The dependence on scan rate with increasing coverage is consistent with previous reports at CoPi-hematite27 and NiBi-modified32 hematite and has been ascribed by Gamelin et al. to the presence of a “kinetic bottleneck” in water oxidation at high loading.27 On the other hand, Hamann and co-workers reported greater cathodic shift in the photocurrent onset and an increase in water oxidation efficiency with increasing CoPi thickness on thin hematite films.34 Gamelin and co-workers attributed differences in the observed dependence on coverage in different studies to the planar versus mesostructure, proposing that increasing coverage at the latter creates recombination sites parallel to the semiconductor/solution interface, reporting optimum CoPi

Figure 3. Cyclic voltammograms at Ni−20/hematite NR film at different positive scan limits 1.8 V (a), 1.2 V (b), 1.0 V (c), and 0.4 V (d). The inset shows the same CVs with the full potential scan range. The initial scan direction is from negative to positive potentials. Electrolyte is 1 M KBi pH 9.2. Scan rate is 100 mV/s.

hematite in 1 M KBi with different positive limits of 0.4, 1.0, 1.2, and 1.8 V, and Figure S4 shows scans to 0.8 and 1.6 V at 200 mV/s. The C2 peak appeared only after scanning to potentials positive of the Ni(OH)2 to NiOOH oxidation into the water oxidation potential (plot c Figure 3), and its current magnitude and charge increased with increasing anodic polarization. The same behavior was observed when the electrode was biased for 20 s at 1.8, 1.5, 1.2, 1.0, 0.7, and 0.4 V, and then the potential was negatively scanned from 0.4 V, as presented in Figure S5. Holding the potential for 20 s at 1.8 V or scanning the potential to 1.8 V led to the same peak magnitude, and biasing longer for 120 s did not affect its magnitude within experimental variation (Figure S5). The peak appeared on the first negative scan and not on subsequent scans (Figure S5). The C2 peak must therefore correspond to the discharge of an oxidized state at Ni-oxide modified-hematite formed at these positive potentials, whose population increases upon increasing anodic polarization in the water oxidation region. It is cathodically shifted by 0.200 V and has a greater magnitude than a peak observed at bare hematite. Its peak current at the same positive limit depended linearly on the scan rate (R2 = 0.99), which is characteristic of a surface process (cf. Figure S6, Ni−25), although this would be complicated by a following reaction. The C2 charge was significantly greater than the A1 and C1 charges at Ni-hematite. This may be attributed to the large surface area of the modified nanorods if C2 corresponds to the electrochemical discharge of an oxidized species in contact with hematite, and the A1/C1 peaks correspond to NiBi in ohmic contact with FTO, as discussed below. Photoelectrochemical Study at Hematite NR Arrays Modified in Ni2+/KBi. Figure 4 presents CVs in the light at hematite before and after modification with Ni−10 at 10 mV/s and a CV in the dark at Ni−10/hematite. The bias required to D


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the presence of a smaller magnitude peak near the photocurrent onset with Epeak at 0.030 V at 100 mV/s and as a smaller shoulder at 10 mV/s. It is noted that the bare hematite electrochemical behavior was stable under illumination in 1 M KBi; Figure S8 presents consecutive CVs at different scan rates first in the light and then in the dark, showing that scanning the potential in the light in the absence of Ni2+ did not affect the electrode. To probe at which potentials the photooxidized species that was discharged in C 2,hν forms at Ni-hematite under illumination, Figure 6 presents photocurrent−voltage curves at Ni−10/hematite scanned to 0.2 V (a), 0.6 V (b), and 1.0 V (c), at 100 mV/s. Presented in Figure 6 are also CVs at Ni−10/ hematite under illumination scanned to 0.6 V then with the light turned off during the reverse scan at ca. 0.54 V (d) and at the bare hematite electrode under illumination scanned to 1.6 V (e). When the illuminated modified electrode potential is scanned to 0.2 V, a small reduction shoulder appeared on the reverse sweep. The (C2,hν) peak magnitude increased with scanning to 0.6 V and then remained almost invariant, even when scanning to 1.6 V (inset of Figure 6). At more positive potentials photooxidation of water becomes limited by the flux of holes at the surface or occurs via unmodified Fe-oxide states. When turning the light off during a negative scan following a forward sweep to 0.6 V, the cathodic peak was almost Gaussian with Epeak at ca. −0.025 V, shifting toward C2 Epeak position (in the dark) as shown in the CVs in the inset of Figure 6. The peak does not similarly appear in the CV under illumination at the unmodified electrode scanned to 1.6 V (Figure 6, scan e). The results indicate that C2,hν corresponds to a majority carrier reduction of a long-lived photooxidized species formed at Nihematite at low potentials positive of Efb, where only minimal steady-state photocurrents were generated at bare hematite, and that its formation must be linked to the cathodic shift in photocurrent onset at Ni-hematite. Photocurrent transient measurements revealed that the modification slows the initial photocurrent decay. I−V curves under continued or chopped illumination before and after modification with Ni−25 are presented in the inset of Figure 5B (at 10 mV/s). A charging photoanodic current is measured when the light is turned on and then decays to a steady state approaching the current under continuous illumination at 10 mV/s. Both the initial charging and steady-state currents near Efb increase as a result of the modification. Figure 7 compares photocurrent transients upon turning the light on and off at hematite before and after modification (Ni−25) biased to the same potential (−0.2, 0, 0.2, and 0.4 V), chosen near the photocurrent onset or where significant water oxidation occurs. The amperometry traces show the initial charging photoanodic current (ih) at the onset of illumination reflecting the initial surface hole concentration, which is a function of band bending, and its decay to a steady state (iss). A decaying cathodic overshot is also observed when turning the light off, corresponding to the neutralization of the remaining photooxidized species. The rate of the photoanodic current decay was followed at times longer than ∼200 ms, therefore, after initial fast e−h recombination, and the observed rate constant is denoted kobs which equals krec + kct, the sum of recombination (krec) and charge transfer (kct) rate constants, according to a general model presented by Peter et al.52 The photocurrent decay from 200 ms after onset toward the steady state53,54 (plot of J(t)−Jss versus t) at 0 V at Ni−25/hematite can be fit to a single exponential decay with kobs of 0.28 s−1 or 0.24 s−1 (R2 =

Figure 5. (A) CVs under illumination at hematite NR array at 10 and 100 mV/s and at Ni−10/hematite at scan rates of 10, 20, 50, 100, and 200 mV/s. The inset of (A) shows the negative sweeps at bare hematite at 100 mV/s (a) and 10 mV/s (b) near the photocurrent onset. (B) CVs under illumination Ni−25/hematite at scan rates of 10, 20, 50, 100, and 200 mV/s and at the unmodified hematite at 10 and 100 mV/s. The inset of (B) shows scans at 10 mV/s at the hematite NR array before and after modification under chopped and continued illumination. Electrolyte is 1 M KBi pH 9.2.

thickness of 2.5 nm on mesostructured hematite.35 The observed scan rate dependence at the Ni-hematite array herein is consistent with the one reported at this electrode (nano)structure.35 CVs in Figure 5 at Ni−10/ and Ni−25/hematite under illumination reveal the presence of a prominent reduction peak on the reverse negative scan with Epeak near the photocurrent onset (e.g., Epeak at −0.074 V or −0.124 V at 100 mV/s and 200 mV/s at Ni−10/hematite). This cathodic peak is termed C2,hν. Scans up to 1.6 V show C2,hν to have similar onset as C2 (dark cathodic peak) but with greater currents at the same scan rate and positive limit (Figure S4) and a negatively shifted peak potential. When the potential was reversed at 0.8 V at Ni−20/ hematite this reduction peak was not present in dark CVs but appeared when the electrode was illuminated (Figure S4, 200 mV/s). Comparing CVs at Ni−10 and Ni−25 on hematite in Figure 5 shows that the C2,hν peak increased in magnitude with increasing time of illumination in Ni2+/KBi. Inspection of I−V curves at illuminated bare hematite (inset of Figure 5A) shows E


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Figure 6. CVs under illumination at Ni−10/hematite scanned to 0.2 V (a), 0.6 V (b), and 1.0 V (c). CV under illumination scanned to 0.6 V then with the light turned off during the reverse scan without stopping the scan (at ca. 0.54 V) (d), and CV under illumination at the untreated hematite scanned to 1.6 V (e). Initial scan direction is from negative to positive potentials. The inset shows scans in the dark and under illumination at this treated array scanned to 1.6 V along with the (d) scan (light on then off). Electrolyte is 1 M KBi pH 9.2. Scan rate is 100 mV/s.

Figure 7. Chronoamperometry photocurrent transients upon turning the light on and off at Ni−25/hematite NR array (a) and before modification (b) biased at −0.2 V (A), 0 V (B), 0.2 V (C), and 0.4 V (D). The inset of B shows a single exponential fit for J(t)−Jss versus time with R2 = 0.99 at 0 V at the Ni−25/hematite film.

s−1 (t1/2 = 135 ms, R2 = 0.966) for the best single-exponential fit. Most of the photogenerated holes recombine at 0 V at

0.998 or 0.991; t1/2 = 2.5 or 2.9 s, respectively), compared to a significantly faster decay before modification with kobs of 5.15 F


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The Journal of Physical Chemistry C hematite, leading to a negligible iss, while a relatively appreciable iss is recorded at Ni-hematite. At Ni−25-hematite at −0.1 V, kobs equaled 0.20 s−1 (t1/2 = 3.4 s, R2 = 0.999) compared to 3.44 s−1 (t1/2 = 202 ms, R2 = 0.88) at the same electrode before modification. At 0.2 V, the photocurrent decay occurs with kobs = 0.54 s−1 (t1/2= 1.3 s; R2 = 0.98) compared to 4.7 s−1 (t1/2 of 147 ms, R2 = 0.92) at the unmodified electrode. At 0.4 V where appreciable water photooxidation occurs, kobs decreased from 6.7 s−1 (t1/2 = 104 ms; R2 = 0.98) at hematite to 0.87 s−1 (t1/2 = 0.80 s; R2 = 0.95) at Ni−25/hematite. The photocurrent decay rate constant of the cathodic overshot is also reduced by the modification. A similar trend was recorded at another hematite NR modified with Ni−10 (Figure S9), although with shorter lifetimes but in the same time scale compared to Ni−25. For instance at −0.1 V, t1/2 equaled 1.5 s at Ni−10/hematite and 3.4 s at Ni−25/hematite. At 0 V, the photocurrent at hematite had a median lifetime of 122 ms (R2 = 0.91) which increased to 1.2 s (R2 = 0.998) at Ni−10/hematite. Rate constants and median lifetimes obtained at different bias at two arrays before and after modification (Ni−25 or Ni−10) are presented in Table S1. kobs was not a function of potential at hematite, possibly because of different dependence of kct and krec on potential; kct has been reported to increase and krec to decrease with increasing potential at hematite.43,52 Durrant and co-workers studied the dynamics of photogenerated holes using TA at hematite modified with Co-oxide and reported a greater yield of the hematite long-lived holes responsible for water oxidation under bias with no evidence of hole transfer to the redox layer,33 proposing that the redox layer effect was to enhance electron depletion or band bending in iron oxide.33 Since an initially greater ih and slower decay can be a result of greater band bending as proposed, it was examined if the slower photocurrent decay at Ni-hematite can also result from increasing bias. Figure S10 presents photocurrent transients at Ni−25/hematite biased at −0.2, 0, and 0.2 V compared to traces at the unmodified arrays biased 0.2 V more positive, showing faster decay at bare hematite under a 0.2 V greater bias, with respective median lifetimes of 147 or 104 ms at 0.2 or 0.4 V, compared to ca. 2.5 s or ca. 1.3 s at 0 and 0.2 V at Ni−25/hematite. The photohole decay was slower at Ni-hematite compared to hematite biased up to 0.6 V more positive (Table S1). Therefore, any enhanced band bending affected by the redox layer cannot alone explain differences in photoelectrochemical behavior resulting from Ni modification of hematite. Upon modifying hematite nanorods in Ni2+/KBi (aq), the greater steady-state photocurrent at lower bias and the cathodic shift in photocurrent onset coincided with the appearance of a significant reduction peak on the reverse scan in CVs at the illuminated photoanode. The peak was Gaussian when the light was turned off and depended linearly on scan rate, characteristic of a surface process. This peak is ascribed to the discharge of a photooxidized species which has not yet led to water oxidation nor recombined in a back-reaction with electrons. The reduction peak was observed after low bias near Efb at illuminated Ni-hematite where steady-state photocurrents are measured. Its coinciding with the photocurrent onset leads to a belief in the involvement of the formation of this photooxidation state in reducing the bias needed for water photooxidation. The similar potentials of C2,hν and C2, the reduction peak that appeared at similar potential when NiBihematite was biased into water oxidation potentials in the dark, point that the photooxidized state could be the same oxidized

species formed in the dark when the modified electrode is biased at oxygen evolution potentials very positive of Efb. The persistence of C2,hν in the light even at slow scan rate reflects the stability of this photooxidized state at NiBi-hematite. Photocurrent transient measurements confirmed a slower photoanodic current decay at the modified hematite nanorods, with decay lifetime on the order of seconds. A longer lifetime can result from a decrease in the driving force for the recombination of electrons with holes in the NiOx-hematite electrodein other words a slower back-electron reaction with the photooxidized species. It is noted that the cathodically shifted peak potential of C2 at Ni-hematite in the dark compared to the bare electrode denotes a harder to reduce oxidized state. The question remains to the chemical identity of the oxidized state involved in water oxidation. The mechanisms of water photooxidation at hematite, or by which surface-coupled OER catalysts such as CoPi enhance this photooxidation, are not fully understood. Studies at hematite and TiO2 have indicated that transfer of a long-lived hole to a water surface-bound or solution species is the rate-determining step, occurring in the millisecond−second time scale.15−17,55 At hematite under anodic bias where water oxidation occurs, a long-lived hole lifetime of ca. 3 s was measured,15−17 its slow decay reflecting slow hole transfer to water.56 Competing with the slow kinetics is fast e−h recombination with different rates reported as fast as the picosecond time scale12−14,18,19,50 and in the microsecond−millisecond time scale.15,16 The hole at hematite is believed to be trapped at the surface16,17,57,58 before transfer to water species.40,41 Hole transfer to this surface state was proposed to lead to oxidation of Fe(III)-hydroxide to Fe(IV)-oxo intermediate, though its chemical identity has not been confirmed.28,40,41,59,60 The slow kinetics has been ascribed to significant Fe(IV) character.8,16 Klahr et al. attributed a 0.6 V greater overpotential for water oxidation compared to a fast redox shuttle [Fe(CN)6]3−/4− to an initial step of holes oxidizing the hematite surface and buildup of surface intermediates before a bimolecular decomposition such as 2Fe(IV)=O → 2Fe(III) + O2.40 Fe(V) and Fe(VI) oxidation states have also been proposed, invoking storage of holes.52 Durrant and co-workers reported a first-order reaction in hole concentration and therefore oxidation via single hole transfer under pulsed laser excitation,17 in agreement with a theoretical study,59 but recently Le Formal et al. showed a transition to third-order reaction at high hole density, showing the possibility of multihole accumulation.61 Hamann, Bisquert, and co-workers observed using impedance spectroscopy a “prominent” surface state where holes accumulate at bare thin hematite, identified by a Gaussian capacitance peak vs potential.40,41 They showed that this charging coincides with the photocurrent onset, and photooxidation of water occurs via this surface state.40,41 The capacitance peak was observed upon biasing bare hematite at 2 V vs RHE under 1 sun to oxidize a state believed to form an intermediate in water oxidation.40,41 The reported peak surface state capacitance (Css) at a 60 nm film equaled 110 μF/cm2 at 1000 mV/s and decreased with lowering scan rate to 60 μF/ cm2 at 200 mV/s, due to state occupancy decaying with time,40 while Css of ∼150−160 μF/cm2 were reported from current transients at 1 sun.40,41 Klahr and Hamann observed a similar peak in CVs at ∼48 nm hematite film near the photocurrent onset, with a peak Css of 300 μF/cm2.62 The cathodic waves that appeared near the photocurrent onset in CVs of Nihematite in the light, or in the dark when its potential was G


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layer, leading to the A1/C1 peaks, while the C2 reduction peak corresponds to the discharge of the catalyst at the modified nanorods. The great charges of the C2 and C2,hν can then be attributed to the surface area of the modified nanorods. Notably, supporting the independence of the electrochemical processes, I−V curves at Ni-hematite where A1 and C1 peaks were not resolved, attributed to very low coverage, still featured the C2 and C2,hν peaks (Figure S15). A hypothesis to still be examined however is the involvement of surface states and whether Ni-oxide could be modifying the existing α-Fe2O3 surface state believed to accept the photohole and mediate water photooxidation at hematite,40,41 stabilizing this “intermediate” state against back electron transfer. The presence of a reduction peak, albeit at smaller magnitude, at bare hematite supports the presence of an electronic state in the gap capable of oxidation, as reported by Hamann and coworkers.40,41 It can be hypothesized that surface states within the gap charged via photohole transfer exhibit decreased recombination rate if the energetics is modified by mixing with Ni-oxide. The cathodic shift in C2 after NiBi modification would be consistent with a harder to reduce oxidized state. Boettcher and co-workers considered the case of a semiconductor with surface states, in contact with an ion-permeable redox layer, and assumed the surface states and the redox layer will be at quasi-equilibrium, based on faster charge transfer between them than water oxidation.66 Accordingly, the surface states and the catalyst share the same Fermi level and are charged together, with a fast catalyst requiring less charging.66 The results reported herein show direct evidence of surface charging in a stable long-lived photo-oxidized state as a result of modification with the redox Ni(Fe)Bi layer, which can correspond to charging of surface states and redox layer but cannot differentiate the chemical involvement of a possible modified Ni−Fe-oxide state on hematite, or Ni(Fe)OOH catalyst, in water oxidation depending on the kinetics at the different surfaces.

scanned positive into the water oxidation region, and at the unmodified hematite to a lower magnitude bear resemblance to this surface capacitance. For comparison to the charge stored in the C2,hν peak at the nanorods, surface capacitances versus potential were calculated by dividing the current density by the scan rate at the illuminated Ni−10-hematite and Ni−25-hematite (Figures S.11 and S.12, CVs in Figure 5) and unmodified electrode (Figure S.13). A peak capacitance of ca. 115 μF/cm2 is thus determined at the illuminated bare nanorods at 100 mV/s, while peak capacitance equaled 2050 μF/cm2 at Ni−10hematite at 100 mV/s or 3600 μF/cm2 or 3120 μF/cm2 at 10 and 100 mV/s, respectively, at Ni−25-hematitethus 10−30fold greater than reported at thin bare hematite40,41 and ∼18fold or 27-fold greater than at the bare nanorods in this study at 100 mV/s. A 113 nm diameter nanorod standing vertically will have a roughness factor (microscopic/projected area) of 19 if 500 nm long or 36 if 1 μm long, and the electrode roughness factor will, for instance, equal ∼16 if a simple square 2D assembly is assumed, which could explain the large capacitance. The significantly smaller cathodic peak at illuminated bare nanorods can be understood by the presence of fast e−h recombination. The chemical identity of the state accepting the hole at NiBihematite and its role in water oxidation warrant further study. Earlier mechanistic studies by Nocera and co-workers showed that oxygen evolution is catalyzed at (anodized) NiBi via a twoelectron, three-proton pre-equilibrium step followed by a chemical rate-determining step,63 and in situ X-ray absorption spectroscopy (XAS) studies showed involvement of a mixed valent NiIII/IV.49 Studies by Boettcher and co-workers have shown however that NiBi in KOH64 or in borate buffer65 does not reach its greater OER activity in Fe-free electrolyte and that doping occurs from trace Fe in the electrolyte and, therefore, that the active catalyst is Ni(Fe)OOH rather than NiOOH. The results of this study can be argued to be consistent with a hypothesis that the holes are transferred to the bulk of the redox layer, resulting in a photooxidized Ni(Fe)OOH in Ni(Fe)Bi that catalyzes water oxidation. This photohole transfer results in greater charge separation, indicated by a lower e−h recombinationdue to slower back-reaction of majority carriers to reduce the photooxidized state. The charge transferred (C2,hν) increased with increasing coverage, also consistent with transfer to the redox layer. In this, a mechanism of hole transfer from hematite to Ni(Fe)Bi could therefore be proposed followed by water oxidation at the catalyst after its Fermi level is anodic enough, in agreement with a picture of hole transfer to CoPi on hematite27,34,35 and Ni-oxide on TiO2.42 It might also be possible that this hole transfer affects to some extent the semiconductor depletion layer, but e−h recombination was found to be slower at the modified nanorods than with 0.6 V more anodic bias at bare nanorods. Some electrochemical observations here need to be reconciled with a Ni(Fe)(OH)2/Ni(Fe)OOH state accepting the hole. The C2 peak charge was greateras great as 10-fold in some measurementsthan the A1/C1 peaks (Qc2/QC1 is shown in Figure S14 at Ni−25), and the A1 oxidation peak of Ni(OH)2/ NiOOH was still resolved following the plateau photocurrent of water oxidation in fast I−V scans at Ni-hematite, indicating the presence of a NiII state in the overall structure of Ni(OH)2/ NiOOH in borate on hematite on FTO biased positive of Efb (seen in Figure 5 in the CVs at 100 or 200 mV/s and in the inset of Figure 6). It may be that some photodeposited NiBi is in contact with FTO directly or through a very thin tunneling

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CONCLUSIONS The effect of modifying hematite nanorod electrodes in Ni2+/ potassium borate on the photoanode electrochemical and photoelectrochemical behavior was investigated. The surface modification was found to result in trapping of the photogenerated hole with a longer lifetime on the order of seconds at low bias. The cathodic shift in photocurrent onset and greater photocurrent at lower bias coincided with the observation of a major cathodic wave in cyclic voltammograms near the photocurrent onset when the electrode was illuminated at potentials positive of Efb, or following its dark oxidation at water oxidation potentials, which provided evidence of hole transfer to a stabilized oxidized state with slow back-electron reaction. Although evidence shows hole transfer in a long-lived oxidized state at the modified surface, the chemical identity of the state that accepts and stabilizes the hole at NiBi-modified hematite that is directly involved in water oxidation is yet to be determined. The effect of these surface modifications on the energetics of the Fe2O3 state believed to be involved in water oxidation at hematite, at the interface in the environment of Co-oxide or Ni-oxide redox layers, remains a question to be investigated, particularly considering the synergistic effect observed at bimetallic Ni−Fe-oxide on water oxidation, in addition to the dependence of the (photo)electrochemical behavior on the surface modification method and electrode structure. H


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EXPERIMENTAL METHODS Materials. The following materials and electrodes were used in this study: iron(III) chloride hexahydrate, FeCl3 (SigmaAldrich, puriss ≥99%); sodium hydroxide pellets, NaOH (MACRON Chemicals, ACS); potassium hydroxide, KOH (Macron Chemicals); sodium nitrate, NaNO3 (Aldrich, ACS reagent); boric acid, H3BO3 (Macron Chemicals, 99.99%); nickel(II) nitrate hexahydrate, Ni(NO3)2 (Strem Chemicals, puratrem, 99.9985%); Pt gauze (Alfa Aesar); Ag/AgCl reference electrode (BASi); fluorine-doped tin oxide, FTO (Hartford Glass Co. Inc., TEC7, 1 × 3 cm2, 2.2 mm thick); and deionized nanopure water (Millipore, resistivity ≥18 Ω.cm). Hematite Nanorod Preparation. Hematite nanorod arrays were prepared on FTO. FTO electrodes were cleaned by sonicating for 10 min in water and 30 min in isopropanol, rinsing with water, and drying under N2(g) flow. Arrays of hematite nanorods were fabricated using a hydrothermal reported procedure44 by immersing FTO substrates in 10 mL of FeCl3 (0.4 g) and NaNO3 (0.85 g) (aq) solution at pH 1.15 and heating at 95° for 4 h in a 23 mL Teflon-lined autoclave (PARR instruments). The pH was adjusted with concentrated HCl and measured with a symphony SP70P pH meter (VWR). Films were thoroughly rinsed with water, dried in air, and annealed at 550 °C for 2 h (Thermolyne Oven), with a heating rate of 10°/min. Films were subjected to a second annealing protocol by introducing to a preheated oven at 550 °C, heating at 10°/min to 800 °C, dwelling for 20 min, and then cooling at the same rate to 550 °C. The films were removed from the oven at 550 °C and allowed to cool to RT. Some softening of the glass results with heating to 800 °C.45 Safety note: Proper safety precautions must be taken when introducing and taking out the f ilms at 550 °C, but this procedure was found to minimize the softening of the glass. Photochemical Modification with NiBi. Hematite electrodes were modified by dipping in a solution of 0.4 mM Ni(NO3)2 in 0.1 M potassium borate buffer (KBi) pH 9.1−9.2 in a glass vial for the indicated period of time with white light from a Xe lamp. In some experiments, films were modified for a specified time t1 and then, following photoelectrochemical measurements, were further modified for another time t2. The films are labeled according to the total time of modification (t = t1 + t2 + ...) as Ni−t only for easy referencing. The anodic and cathodic peaks corresponding to NiII/III redox couple are taken as a measure of the deposited NiBi amount, as the illumination intensity during photodeposition was different in different experiments. Electrochemical and Photoelectrochemical Measurements. Electrochemical and photoelectrochemical measurements were collected using a model CHI760C electrochemical workstation in a 3-electrode cell using a Pt gauze as the counter electrode and Ag/AgCl as a reference electrode in 1 M KBi. The hematite film was sealed with an exposed area of ca. 1 cm2. Measurements were acquired at the same film before and after modification. The electrochemical cell consisted of a twocompartment H-cell with a medium porosity frit having a separate compartment for the auxiliary electrode. The films were illuminated from a 300 W Xe lamp (69911 power supply and 67005 lamp housing, Newport) and an AM 1.5 filter (Newport) with a light intensity of 100 mW/cm2. The light was manually chopped. All films were illuminated from the frontwall side (electrolyte side). The light intensity was measured using a Coherent power meter with a model LM3

power head. Potentials are reported relative to Ag/AgCl. Potentials are calculated where indicated versus the RHE as follows: EvsRHE (V) = EvsAg/AgCl (V) + 0.197 V + (0.059 V) pH. The pH of the 1 M borate buffer was between 9.2 and 9.27. SEM and UV−Visible Spectroscopy. SEM imaging was acquired using a field-emission SEM (Philips XL, ESEM-FEG, MIT). Absorbance spectra were acquired with a Cary 5000 UV−vis−NIR spectrophotometer. Energy-dispersive X-ray (EDX) spectra were acquired using TESCAN, MIRA 3 LMU, FEG SEM (CRSL, AUB) with OXFORD instruments EDX detector INCA X-MAX20.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04430. Absorbance spectra of hematite NRs, EDX spectra of NiBi-modified hematite, CVs of NiBi on FTO, CVs at Ni/hematite in the dark and under illumination with positive limits of 1.6 and 0.8, CVs starting in the negative direction from 0.4 V at Ni/hematite electrode held for 20 s at different positive limits from 1.8 to 0.4 V and comparison with biasing for 120 s or with scanning the potential on the cathodic peak C2, CVs at Ni/hematite in the dark at different scan rates and dependence of the C2 cathodic peak current on scan rate, CVs showing the scan rate dependence of I−V curves under illumination at Ni/ hematite illuminated for 30 min showing the effect of increasing coverage, consecutive CVs at bare hematite in the dark and under illumination at different scan rates, photocurrent transients at hematite compared to Ni−10/ hematite at different bias, photocurrent transients comparing Ni−25/hematite to hematite with 0.2 V more positive bias, photocurrent decay rate constants and halflife for two independently prepared hematite films and then after modification with Ni−10 or Ni−25, capacitance versus potential plots at Ni−10/hematite and Ni−25/ hematite and unmodified hematite for C2,hν from CVs under illumination at different scan rates, ratio of the cathodic peak charges C2 to C1 at different scan rates, and CVs at Ni/hematite with low coverage not showing the A1/C1 peaks (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Phone: 9611350000/ext.3996. Notes

The author declares no competing financial interest. § Permanent address is American University of Beirut. Author was a visiting scholar, on sabbatical, at Massachusetts Institute of Technology.

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ACKNOWLEDGMENTS LH acknowledges support for a sabbatical research leave funded by the American University of Beirut (AUB) in spring and summer 2012 that she spent as a visiting scholar at MIT where she conducted the experiments; data analysis and writing of the manuscript were conducted at AUB. LH thanks Professor Daniel Nocera for hosting her in his laboratory at MIT during her sabbatical and generously allowing her to conduct this independent research on the subject, and she thanks him and members of his group for their useful comments on the I


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manuscript. LH thanks Ms N. Beydoun for collecting the EDX spectrum (AUB, CRSL) at the time of the review of this paper and Mr. J. Younes at CRSL for assistance in this measurement.

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