Photoelectrochemical Properties and Stability of Nanoporous p-Type

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Photoelectrochemical Properties and Stability of Nanoporous p‑Type LaFeO3 Photoelectrodes Prepared by Electrodeposition Garrett P. Wheeler and Kyoung-Shin Choi* Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: LaFeO3 is a p-type oxide that has an ideal bandgap and band edge positions for overall solar water splitting. This study reports an electrochemical synthesis method to produce LaFeO3 as a high surface area, nanoporous photocathode. The resulting electrode generated a photocurrent density of −0.1 mA/cm2 at a potential as positive as 0.73 V vs RHE for photoelectrochemical oxygen reduction with a photocurrent onset potential very close to its flatband potential of 1.45 V vs RHE. Furthermore, without a protection layer, it showed stable photocurrent generation with no sign of photocorrosion. Due to the poor catalytic nature of the LaFeO3 surface for water reduction, the photocurrent obtained for water reduction was not substantial. However, its photostability and its ability to achieve a photovoltage for water reduction greater than 1.2 V encourage further studies on doping to enhance electron−hole separation as well as interfacing appropriate hydrogen evolution catalysts. ∼400 mV more negative than what was shown by the LaFeO3 photocathode prepared by the sol−gel spin-coating method.9 In this study, we report an electrochemical synthesis method for the preparation of high surface area nanoporous LaFeO3 photocathodes. Because the nanoporous morphology significantly reduces the distance that the minority carriers need to travel to reach the electrode/electrolyte interface, both a very positive photocurrent onset potential and enhanced photocurrent density could be achieved. Furthermore, the photostability of LaFeO3 could be confirmed using the nanoporous LaFeO3 photocathodes. The nanoporous LaFeO3 photocathodes used in this study were prepared by electrochemically codepositing La(OH)3 and Fe(OH)2 by nitrate reduction (see the Supporting Information for experimental details).10,11 The plating solution contained La3+ and Fe2+ ions as well as nitrate ions. When a potential is applied to the working electrode to reduce nitrate to nitrite in an aqueous solution, OH− is generated, causing a local pH increase at the surface of the working electrode (eq 1).

P

erovskite-type lanthanum iron oxide, LaFeO3, is a p-type semiconductor that has several attractive features for use as a photocathode in a water splitting photoelectrochemical cell (PEC). Its bandgap energy is reported to be ∼2.1 eV, allowing for the utilization of a large portion of the visible solar spectrum.1−3 At the same time, LaFeO3 synthesized as a powder-type photocatalyst demonstrated the capability of generating both H2 and O2 under illumination. Semiconductors that have a bandgap energy of ∼2.1 eV while having their conduction band and valence band positions suitable for both water reduction and oxidation are extremely rare. Furthermore, LaFeO3 is one of the few p-type oxides that do not contain copper. As the presence of copper in most p-type oxides is known to be the cause for photocorrosion, LaFeO3 has the possibility of being a photostable p-type oxide.4−6 LaFeO3 has been mainly studied as a powder-type photocatalyst for solar water splitting and dye degradation.1−3,7 There have been only two reports that investigated LaFeO3 as a photocathode for a water splitting PEC. The LaFeO 3 photocathode prepared by a sol−gel spin-coating method showed a photocurrent onset potential that is very close to its flatband potential of 1.4 V vs RHE, which is highly desirable.8 However, it generated limited photocurrent density. The LaFeO3 photocathode prepared by pulsed laser deposition generated a higher level of photocurrent density, but its photocurrent onset potential was ∼1.0 V vs RHE, which is © XXXX American Chemical Society

NO−3 + H 2O + 2e− → NO−2 + 2OH−

(1)

Received: July 21, 2017 Accepted: September 12, 2017

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters As a result, the solubilities of La3+ and Fe2+ on the surface of the electrode are decreased, and these ions are precipitated as La(OH)3 and Fe(OH)2. After precipitation, when no longer protected by the cathodic bias, Fe(OH)2 is further oxidized to FeOOH in the air. The ratio of La(OH)3 and FeOOH in the deposited film can be adjusted by changing the concentration of La3+ and Fe2+ in the plating solution. A plating solution containing 20 mM La(NO3)3·6H2O and 18 mM FeCl2·4H2O as well as 400 mM KNO3 was identified to be optimum to deposit films containing a 1:1 ratio of La(OH)3 and FeOOH, which was estimated by energy-dispersive spectroscopy (EDS). The as-deposited films were annealed at 600 °C for 3 h in air to form crystalline LaFeO3. X-ray diffraction (XRD) patterns of the as-deposited and annealed La−Fe−O films are shown in Figure 1. The as-

Figure 2. Top-view SEM images of (a) an as-deposited La−Fe−O film and (b,c) an annealed LaFeO3 film. (d) Side-view image of an annealed LaFeO3 film.

Figure 1. XRD patterns of the as-deposited film (black) and the post-annealed LaFeO3 film (blue). The (hkl) indices are based on JCPDS 37-1493 (space group: Pnma with a = 5.5669(4) Å, b = 7.8547(7) Å, and c = 5.5530(8) Å). The peaks from the FTO substrate are indicated by an asterisk. Figure 3. (a) UV−vis−NIR absorption spectrum of LaFeO3 with a Tauc plot in the inset and (b) Mott−Schottky plots of LaFeO3 measured in 0.1 M NaOH (pH 13).

deposited film is amorphous and shows only FTO substrate peaks. However, the annealed film is crystalline and shows diffraction peaks whose positions and relative intensities match well with those of orthorhombic LaFeO3 (JCPDS No: 371493) with no impurity peaks present.12 The SEM image of the as-deposited La−Fe−O film is shown in Figure 2a. It is composed of a high surface area nanoporous network. Upon annealing, the nanoporous network contracted slightly due to the dehydration of La(OH)3 and FeOOH, creating more void space in the nanoporous network (Figure 2b). A high-magnification SEM image shows that the average size of the nanoparticles that compose the LaFeO3 electrode is less than 50 nm (Figure 2c). This agrees well with the average particle size calculated from XRD using the Scherrer equation, which is 33 nm. A side-view SEM image shows uniform coverage of the FTO surface with LaFeO3 (Figure 2d). The film thickness was estimated to be 1.5 μm and was experimentally determined to be the optimum thickness for photoelectrochemical performance. The UV−vis−NIR absorption spectroscopy of LaFeO3 shows a well-defined absorption onset due to the bandgap transition at around 575 nm. Its bandgap energy was estimated to be 2.16 eV assuming a direct bandgap (Figure 3a).1−3 Mott−Schottky plots of LaFeO3 electrodes were obtained in 0.1 M NaOH (pH 13) at two different frequencies and are shown in Figure 3b. The negative slopes confirm the p-type nature of the LaFeO3 electrode. The slopes show a frequency

dependence, which is commonly observed with nanocrystalline films. However, the x-intercepts converge at 1.45 V vs RHE, which is interpreted as the flatband potential of LaFeO3. The flatband potential of the nanoporous LaFeO3 electrode is comparable to that of the LaFeO3 electrode prepared by a sol− gel spin-coating method.8 The carrier density of the nanoporous LaFeO3 electrodes could not be determined from the Mott−Schottky plots because of the nanocrystalline nature of the LaFeO3 electrode and the frequency dependence of the slopes as well as the lack of an exact surface area of the electrode. Therefore, we could not determine the exact location of the valence band maximum (VBM). However, if we assume that the flatband potential is ∼200 mV above the valence band edge, which can be a reasonable assumption for a lightly doped semiconductor,13 combining it with the bandgap of 2.16 eV, the conduction band minimum (CBM) of the nanoporous LaFeO3 photocathode is estimated to be ∼−0.5 V vs RHE. The photoelectrochemical properties of LaFeO3 electrodes were first examined for O2 reduction in O2-saturated 0.1 M NaOH (pH 13). The photocurrent for water reduction by bare LaFeO3 is expected to be significantly limited by the interfacial electron transfer reaction because LaFeO3 is not catalytic for water reduction. However, LaFeO3 is known to be electrocatalytic for O2 reduction.14 Therefore, photocurrent measure2379

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ACS Energy Letters ment for O2 reduction can allow us to obtain photocurrent of LaFeO3 that is not completely restricted by the interfacial electron transfer reaction. Figure 4a shows the J−V plot of LaFeO3 for oxygen reduction obtained under AM 1.5G 100 mW/cm2 illumination.

to date, it is still far less than what is expected by the bandgap energy of LaFeO3. Because the absence of the transient photocurrent suggests no loss due to surface recombination, we believe that the major limiting factor of our nanoporous LaFeO3 photocathodes is low electron−hole separation caused by bulk recombination. We are currently investigating methods to improve the majority carrier concentration and hole transport properties of LaFeO3 to improve electron−hole separation. The J−t plot for O2 reduction was measured at 0.75 V vs RHE (Figure 4b). We note that this potential, which was chosen to generate an appreciable amount of photocurrent, is more negative than the electrochemical onset potential for O2 reduction. Therefore, a small amount of dark current (∼ −10 μA/cm2) due to electrochemical reduction of O2 was present before the illumination initiated. LaFeO3 generated a stable photocurrent density of −95 μA/cm2 over the course of 1 h. This performance, which is achieved without placing any protection layer, looks highly promising. For comparison, photocurrent generated by typical p-type copper oxides such as Cu2O or CuO photocorrode rapidly when no protection layer is present.4,15,16 The J−V plot of LaFeO3 photoelectrodes was also measured for water reduction under AM 1.5G 100 mW/cm2 illumination using a closed cell containing N2-purged 0.1 M NaOH (pH 13) with a small gas inlet and outlet, through which N2 was constantly purged. Because LaFeO3 is highly catalytic for O2 reduction,14 even a trace amount of O2 present in solution can enhance photocurrent generation. We found that even if a N2saturated solution was used and N2 was constantly bubbled during the measurement, O2 from the air could still be dissolved into the electrolyte when an open cell was used and the solution was stirred. For this situation, photocurrent obtained by LaFeO3 for water reduction was not solely due to water reduction and was always higher than photocurrent measured for water reduction using an airtight cell. This means that falsely enhanced photocurrent for water reduction can be obtained when an open cell is used with LaFeO3 photocathodes. The J−V plot obtained for water reduction using chopped light shows large transient photocurrent (Figure 5a), which indicates significant surface recombination due to poor water reduction kinetics at the surface. This result is expected because LaFeO3 is not catalytic for H2 production. However, a very positive photocurrent onset potential, 1.27 V vs RHE, was still achieved for water reduction due to the nanoporous morphology. The photocurrent onset potential for water reduction measured against RHE is the same as the photovoltage achieved for H2 production (i.e., the difference between the photocurrent onset potential and thermodynamic H2 production potential). The photovoltage for H2 production achieved by LaFeO3 (1.27 V) is significant considering that those of p-type silicon and Cu2O are around 0.4−0.6 V vs RHE.4,17−19 When a proper hydrogen evolution catalyst is added, the photocurrent onset of LaFeO3 for H2 production should be further shifted close to 1.45 V vs RHE, its flatband potential. The J−t plot of the nanoporous LaFeO3 photocathode for water reduction was collected at 0.5 V vs RHE in 0.1 M NaOH (pH 13). It showed a significant transient photocurrent; the initial photocurrent density of −35 μA/cm2 was decreased to −1.5 μA/cm2 over the course of 2 min (Figure 5b). This is again due to the poor water reduction ability of the LaFeO3

Figure 4. (a) J−V plot and (b) J−t plot at 0.75 V vs RHE of LaFeO3 for oxygen reduction under AM 1.5G 100 mW/cm2 illumination in 0.1 M NaOH (pH 13) saturated with O2.

Chopped illumination was used to obtain both the dark current and photocurrent simultaneously using a single scan while also examining the presence of transient photocurrent. Because LaFeO3 is catalytic for O2 reduction,14 the dark current of the J−V plot shown in Figure 4a is not flat but shows a reduction wave initiating at around 0.9 V vs RHE due to electrochemical reduction of O2. The J−V plots showing only the dark currents obtained in an O2-saturated solution and a N2-saturated solution, which can be used to accurately determine the electrochemical onset potential for O2 reduction by LaFeO3, are shown in Figure S1, where the onset potential is determined to be 0.93 V vs RHE. The inset of Figure 4a shows that the photocurrent onset potential is 1.41 V vs RHE, which is very close to the flatband potential. (The nonzero dark current shown in the inset (∼ −2 μA/cm2) is due to charging current associated with sweeping the potential to the negative direction.) This proves that the nanoporous morphology helps electrons reach the surface even when the applied potential is very near the flat band potential. The J−V plot does not show the presence of transient photocurrent for the entire potential region. This confirms that the surface of the nanoporous LaFeO3 electrode used in this study is catalytic for O2 reduction, rapidly consuming all surface-reaching electrons for O2 reduction and suppressing surface recombination. The photocurrent density of the nanoporous LaFeO3 cathode starts to exceed −100 μA/cm2 at a potential as positive as 0.73 V vs RHE. We note that, although this is the best performance demonstrated by LaFeO3 2380

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water reduction, no sign of photocorrosion was observed. This confirmed that LaFeO3 is a rare p-type oxide that does not suffer from photocorrosion. We expect that with systematic doping studies and interfacing a proper hydrogen catalyst, the performance of LaFeO3 can be further improved.

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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/acsenergylett.7b00642. Experimental details and supplementary J−V plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kyoung-Shin Choi: 0000-0003-1945-8794 Notes

The authors declare no competing financial interest.

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Figure 5. (a) J−V plot and (b) J−t plot at 0.5 V vs RHE of LaFeO3 for water reduction under AM 1.5G 100 mW/cm2 illumination in 0.1 M NaOH (pH 13). (The J−V plot obtained under constant illumination is available in Figure S2.)

ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DESC0008707.

surface, allowing the majority of the surface-reaching electrons to be consumed via surface recombination. However, even though the surface-reaching electrons could not be quickly consumed for the interfacial electron transfer reaction (H2 production), the nanoporous LaFeO3 electrode did not show any sign of photocorrosion (e.g., dissolution loss, discoloration, or new phase formation) after the 1 h J−t measurement. This suggests either that LaFeO3 is thermodynamically photostable or that the rate of photocorrosion is significantly slower than the rate of surface recombination. This means that no special treatment (e.g., adding a protection layer) is necessary to prevent photocorrosion of LaFeO3 during water reduction, which is advantageous. When an appropriate hydrogen evolution catalyst is added to make the rate of interfacial electron transfer faster than that of surface recombination, it will be possible to utilize all of the surface-reaching electrons for water reduction. In summary, we developed synthesis conditions to prepare high surface area nanoporous LaFeO3 photocathodes via electrochemical co-deposition of La(OH)3 and Fe(OH)2 followed by an annealing process. The bandgap, flatband potential, and band edge positions of the nanoporous LaFeO3 photocathode were confirmed to be ideal for solar water splitting. When first investigated for photoelectrochemical O2 reduction, the nanoporous LaFeO3 photocathode showed a photocurrent onset potential of 1.41 V vs RHE and generated a photocurrent density of −95 μA/cm2 at a potential as positive as 0.75 V vs RHE in a stable manner with no sign of photocorrosion. Its performance for water reduction was not impressive due to its extremely poor catalytic ability for water reduction. However, a very positive photocurrent onset potential (1.27 V vs RHE) was still achieved for water reduction due to its nanoporous features. Furthermore, after 1 h of J−t measurement for water reduction at 0.5 V vs RHE with a negligible portion of surface reaching electrons being used for

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