Ultra‐Narrow Depletion Layers in a Hematite Mesocrystal‐Based Photoanode for Boosting Multihole Water Oxidation

Abstract

Significant charge recombination that is difficult to suppress limits the practical applications of hematite (α‐Fe2O3) for photoelectrochemical water splitting. In this study, Ti‐modified hematite mesocrystal superstructures assembled from highly oriented tiny nanoparticle (NP) subunits with sizes of ca. 5 nm were developed to achieve the highest photocurrent density (4.3 mA cm−2 at 1.23 V vs. RHE) ever reported for hematite‐based photoanodes under back illumination. Owing to rich interfacial oxygen vacancies yielding an exceedingly high carrier density of 4.1×1021 cm−3 for super bulk conductivity in the electrode and a large proportion of ultra‐narrow depletion layers (<1 nm) inside the mesoporous film for significantly improved hole collection efficiency, a boosting of multihole water oxidation with very low activation energy (Ea=44 meV) was realized.

Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping

“…5b). We note that the excellent sulfite oxidation performances of the phosphorus-doped BiVO 4 samples at low applied potentials (≤1.0 V RHE ) are not only better than those of the state-of-theart BiVO 4 photoanodes (Supplementary Table 1) but also superior to those of other metal oxide photoanodes to date 33,34 .…”

Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping

“…5b). We note that the excellent sulfite oxidation performances of the phosphorus-doped BiVO 4 samples at low applied potentials (≤1.0 V RHE ) are not only better than those of the state-of-theart BiVO 4 photoanodes (Supplementary Table 1) but also superior to those of other metal oxide photoanodes to date 33,34 .…”

Alleviating Charge Recombination Caused by Unfavorable interaction of P and Sn in Hematite for Photoelectrochemical Water Oxidation

“…All hematite samples exhibited characteristic (110) and (300) peaks without the emergence of new phases, suggesting that the electrical conductivityan essential factor for carrier transport in PEC water splittingmight be compromised in the codoped samples. , Mott–Schottky measurements, detailed in Figure f, offered additional clarity on the capacitance changes, flat-band potential (E FB ), and carrier density (N d ) of the samples . These plots, all showing a positive slope indicative of n-type semiconductors, highlighted that P:Sn–Fe 2 O 3 had a significantly lower carrier density than anticipated.…”

“…36,37 Mott−Schottky measurements, detailed in Figure 2f, offered additional clarity on the capacitance changes, flat-band potential (E FB ), and carrier density (N d ) of the samples. 38 These plots, all showing a positive slope indicative of n-type semiconductors, 39 highlighted that P:Sn−Fe 2 O 3 had a significantly lower carrier density than anticipated.…”

“…Furthermore, NiFeO x /P:Sn:Ti−Fe 2 O 3 displayed excellent durability for 30 h in a 1 M NaOH solution (Figure 5d), except for the anodic shift of the onset potential due to an increase in temperature (Figure S26). 39 We calculated the Faradaic efficiency by measuring evolved O 2 and H 2 gases from the NiFeO x /P:Sn:Ti−Fe 2 O 3 photoanode and Pt mesh (cathode), respectively, using an H-type cell at 1.23 V RHE under AM 1.5 G illumination in a 1 M NaOH solution. As shown in Figure 5e, the quantities of evolved H 2 and O 2 gases are 228.4 μmol and 109.7 μmol, respectively, corresponding to the 2:1 ratio of water splitting.…”

Mechanisms of water oxidation on heterogeneous catalyst surfaces

“…Patzke's group recently observed the 3 rd -order reaction kinetics on hematite photoanode in the presence of two surface states at 0.9 and 1.3 V, respectively, which was later interpreted as that the O-O formation was the RDS [65]. In their other two works on hematite, they observed 2 nd order kinetics on bare hematite nanorods array [66] but 3 rd order kinetics on mesocrystal-based hematite photoanode [67]. For the 4 th -order reaction, it is only reported on Ni/Fe oxyhydroxide films (Fig.…”

Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping

“…5b). We note that the excellent sulfite oxidation performances of the phosphorus-doped BiVO 4 samples at low applied potentials (≤1.0 V RHE ) are not only better than those of the state-of-theart BiVO 4 photoanodes (Supplementary Table 1) but also superior to those of other metal oxide photoanodes to date 33,34 .…”

Alleviating Charge Recombination Caused by Unfavorable interaction of P and Sn in Hematite for Photoelectrochemical Water Oxidation

“…All hematite samples exhibited characteristic (110) and (300) peaks without the emergence of new phases, suggesting that the electrical conductivityan essential factor for carrier transport in PEC water splittingmight be compromised in the codoped samples. , Mott–Schottky measurements, detailed in Figure f, offered additional clarity on the capacitance changes, flat-band potential (E FB ), and carrier density (N d ) of the samples . These plots, all showing a positive slope indicative of n-type semiconductors, highlighted that P:Sn–Fe 2 O 3 had a significantly lower carrier density than anticipated.…”

“…36,37 Mott−Schottky measurements, detailed in Figure 2f, offered additional clarity on the capacitance changes, flat-band potential (E FB ), and carrier density (N d ) of the samples. 38 These plots, all showing a positive slope indicative of n-type semiconductors, 39 highlighted that P:Sn−Fe 2 O 3 had a significantly lower carrier density than anticipated.…”

“…Furthermore, NiFeO x /P:Sn:Ti−Fe 2 O 3 displayed excellent durability for 30 h in a 1 M NaOH solution (Figure 5d), except for the anodic shift of the onset potential due to an increase in temperature (Figure S26). 39 We calculated the Faradaic efficiency by measuring evolved O 2 and H 2 gases from the NiFeO x /P:Sn:Ti−Fe 2 O 3 photoanode and Pt mesh (cathode), respectively, using an H-type cell at 1.23 V RHE under AM 1.5 G illumination in a 1 M NaOH solution. As shown in Figure 5e, the quantities of evolved H 2 and O 2 gases are 228.4 μmol and 109.7 μmol, respectively, corresponding to the 2:1 ratio of water splitting.…”

Mechanisms of water oxidation on heterogeneous catalyst surfaces

“…Patzke's group recently observed the 3 rd -order reaction kinetics on hematite photoanode in the presence of two surface states at 0.9 and 1.3 V, respectively, which was later interpreted as that the O-O formation was the RDS [65]. In their other two works on hematite, they observed 2 nd order kinetics on bare hematite nanorods array [66] but 3 rd order kinetics on mesocrystal-based hematite photoanode [67]. For the 4 th -order reaction, it is only reported on Ni/Fe oxyhydroxide films (Fig.…”

Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping

“…5b). We note that the excellent sulfite oxidation performances of the phosphorus-doped BiVO 4 samples at low applied potentials (≤1.0 V RHE ) are not only better than those of the state-of-theart BiVO 4 photoanodes (Supplementary Table 1) but also superior to those of other metal oxide photoanodes to date 33,34 .…”

Alleviating Charge Recombination Caused by Unfavorable interaction of P and Sn in Hematite for Photoelectrochemical Water Oxidation

“…All hematite samples exhibited characteristic (110) and (300) peaks without the emergence of new phases, suggesting that the electrical conductivityan essential factor for carrier transport in PEC water splittingmight be compromised in the codoped samples. , Mott–Schottky measurements, detailed in Figure f, offered additional clarity on the capacitance changes, flat-band potential (E FB ), and carrier density (N d ) of the samples . These plots, all showing a positive slope indicative of n-type semiconductors, highlighted that P:Sn–Fe 2 O 3 had a significantly lower carrier density than anticipated.…”

“…36,37 Mott−Schottky measurements, detailed in Figure 2f, offered additional clarity on the capacitance changes, flat-band potential (E FB ), and carrier density (N d ) of the samples. 38 These plots, all showing a positive slope indicative of n-type semiconductors, 39 highlighted that P:Sn−Fe 2 O 3 had a significantly lower carrier density than anticipated.…”

“…Furthermore, NiFeO x /P:Sn:Ti−Fe 2 O 3 displayed excellent durability for 30 h in a 1 M NaOH solution (Figure 5d), except for the anodic shift of the onset potential due to an increase in temperature (Figure S26). 39 We calculated the Faradaic efficiency by measuring evolved O 2 and H 2 gases from the NiFeO x /P:Sn:Ti−Fe 2 O 3 photoanode and Pt mesh (cathode), respectively, using an H-type cell at 1.23 V RHE under AM 1.5 G illumination in a 1 M NaOH solution. As shown in Figure 5e, the quantities of evolved H 2 and O 2 gases are 228.4 μmol and 109.7 μmol, respectively, corresponding to the 2:1 ratio of water splitting.…”

Mechanisms of water oxidation on heterogeneous catalyst surfaces

“…Patzke's group recently observed the 3 rd -order reaction kinetics on hematite photoanode in the presence of two surface states at 0.9 and 1.3 V, respectively, which was later interpreted as that the O-O formation was the RDS [65]. In their other two works on hematite, they observed 2 nd order kinetics on bare hematite nanorods array [66] but 3 rd order kinetics on mesocrystal-based hematite photoanode [67]. For the 4 th -order reaction, it is only reported on Ni/Fe oxyhydroxide films (Fig.…”