Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22272−22277
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Oxygen-Vacancy-Dominated Cocatalyst/Hematite Interface for Boosting Solar Water Splitting Lei Wang,*,†,‡,§ Jie Zhu,† and Xianhu Liu§ †
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College of Chemistry and Chemical Engineering and Inner Mongolia Key Lab of Nanoscience and Nanotechnology, Inner Mongolia University, Hohhot 010021, China ‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation, National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China § Key Laboratory of Materials Processing and Mold, Ministry of Education, Zhengzhou University, Zhengzhou 450002, China S Supporting Information *
ABSTRACT: Surface suppression is one of critical issues for semiconductors in photoelectrochemical (PEC) water splitting. Deposition of oxygen evolution cocatalysts on photoanodes can improve the oxygen evolution rate, but still it has some limits in some cases. In this work, we propose a new and simple precipitation approach to transform the surface of hematite into iron phosphate (Fe−Pi). Further, Ar-plasma treatment on Fe−Pi/Fe2O3 introduces oxygen vacancies on the phosphorous and photoanode. A surface phosphate treatment accelerates the transfer of holes from the bulk to the surface. Besides, creating oxygen vacancy defects on Fe−Pi/Fe2O3 can significantly increase the reactivity of active sites, leading to the remarkable enhancement in oxygen evolution reaction activity and PEC performance. The resulting photoanode has a current density of 2.71 mA cm−2 at 1.23 VRHE and 3.5 mA cm−2 at 1.50 VRHE under simulated solar light condition. The reduced surface recombination by Fe−Pi layer and Ar-plasma treatment is confirmed by electrochemical analysis. These findings give a great potential of the use of a combination strategy for cocatalyst deposition and optimizing the performance of hematite. KEYWORDS: hematite, plasma-induced defects, Fe−Pi, oxygen vacancies, photoelectrochemical water splitting over, Yu et al.18 proposed a strategy to transform the surface of Fe2O3 into amorphous ion phosphate, where the oxygen atoms were “covalently fixed” in phosphate. The oxygen vacancies were decreased, and the surface states were suppressed. Oxygen vacancies in the optimal concentration range, acting as shallow donor dopants in semiconductors, are favorable for solar water splitting.19,20 Thermal annealing under oxygendeficient atmosphere of Fe2O3 was thought to be an efficient method to control the oxygen vacancy density. The oxygen content affected the formation of oxygen vacancies (Fe2+) and thus the photoactivity of hematite for water oxidation.20−22 Recently, a low-temperature plasma technique to control the oxygen vacancies was applied in the photocatalysis. For instance, the incorporation of oxygen vacancies in ultrathin hematite by an air plasma treatment resulted in an increased carrier density for the improved oxygen evolution reaction activity of photoanodes.23 Additionally, oxygen vacancies induced more active sites in cocatalysts; these have remarkable influence on the oxygen evolution reaction (OER).3,24−28 Wang et al.24 successfully filled the in situ generated oxygen vacancies of Co3O4 with P atom by treating Co3O4 with Ar
1. INTRODUCTION Photoelectrochemical (PEC) water splitting provides an attractive method to convert solar energy into chemical energy in the form of storable hydrogen.1−4 Hematite (Fe2O3) is one of ideal candidate photoelectrodes for a PEC system because its an earth-abundant and stable material, has appropriate band gap (2.0−2.4 eV) for visible light absorption, and has high theoretical solar-to-hydrogen conversion.5−8 However, it has several shortcomings that limit the PEC performance. For example, the low conductivity limits the carrier transportation to the surface; the electron−hole pairs quickly recombine due to the short hole diffusion length (2−4 nm), and the photoinduced hole suffers from poor oxygen evolution reaction (OER) kinetics.8−10 Recently, various strategies (e.g., doping, passivation layers, cocatalysts) have been introduced in hematite to overcome these disadvantages.11−15 The use of dopants in hematite has been widely used to enhance the conductivity and the PEC performance. The nonmetal phosphorous is an excellent dopant to improve its performance. Chen et al.16 reported that phosphorous into hematite bulk resulted in a remarkable enhancement in the PEC activity. Gong and co-workers17 confirmed that the gradient phosphorous incorporation increased the width of band bending over a large region in Fe2O3 for promoting the charge separation efficiency. More© 2019 American Chemical Society
Received: March 1, 2019 Accepted: June 4, 2019 Published: June 17, 2019 22272
DOI: 10.1021/acsami.9b03789 ACS Appl. Mater. Interfaces 2019, 11, 22272−22277
Research Article
ACS Applied Materials & Interfaces plasma in the presence of a P precursor. P−Co3O4 exhibited superior electrocatalytic activities for OER among the best nonprecious metal catalysts. Herein, we fabricated an iron phosphate (Fe−Pi) layer on hematite by simple chemical precipitation, followed by Arplasma treatment. The photoanodes with such a treatment exhibited significantly improved photocurrent. A surface phosphate treatment (Fe−Pi) accelerates transfer of holes from the bulk to the surface. Besides, creation of defects (oxygen vacancies) on the cocatalysts and photoanodes by Arplasma treatment can significantly increase the reactivity of active sites, resulting in the remarkably enhanced OER activity and PEC performance. This work provides a simple and effective strategy for enhancing the photoinduced hole transport to the surface for water oxidation reaction.
2. EXPERIMENTAL SECTION 2.1. Preparation of Fe2O3, Fe−Pi/Fe2O3, and Vo-Fe−Pi/VoFe2O3. Thermal oxidation approach was used for the formation of Fe2O3 nanoflakes (NFs) based on the previous literatures.29,30 The iron foil was immersed in a 5 mM aqueous HAuCl4 solution for 1 min, followed by annealing at 400 °C for 2−3 h. The as-prepared Fe2O3 samples were then immersed in a 5 mM NaH2PO4 aqueous solution for different times and dried with N2 gas (Fe−Pi/Fe2O3). The Fe−Pi/Fe2O3 samples were treated in an Ar plasma (PDC-36G, Kejing) for different times (Vo-Fe−Pi/Vo-Fe2O3). The Fe2O3 NFs were also treated in Ar plasma for comparison. Three microliters of NaH2PO4 aqueous solution was dipped on the surface of Fe2O3 (PO43−/Fe2O3). P-doped Fe2O3 NFs were prepared by annealing PO43−/Fe2O3 at 500 °C. 2.2. Electrochemical Measurements. PEC performances were carried out using a CHI760E electrochemical workstation in a threeelectrode configuration, in which Ag/AgCl (3 M KCl), Pt mesh, and sample worked as the reference, counter, and working electrodes, respectively. A solar simulator with a filter (PLS-FX300HU, PerfectLight, Beijing) was used for simulated AM 1.5G illumination (100 mW cm−2). One molar KOH solution was used as an electrolyte with a pH of 13.6. The scan rate for the linear sweep voltammogram curves was 10 mV s−1. Incident photon-to-current efficiency (IPCE) measurements were performed under monochromatic light by a Xe lamp providing illumination through a monochromator. The electrochemical impedance spectroscopy (EIS) was conducted at a frequency range from 100 kHz to 0.01 Hz using a potentiostat in light. Mott−Schottky (M−S) plots were measured with alternating current frequency of 1000 Hz in the dark. The cyclic voltammetry was performed in 1 M KOH at different scan rates. 2.3. Physical Characterization. The morphology and nanostructure were obtained using a scanning electron microscopy (SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, EITechai G2 20 S-TWIN), respectively. X-ray photoelectron spectroscopy (XPS) was achieved by ESCALAB 250Xi, ThermoFisher machine (ESCALAB 250XI) using Al Kα source. The X-ray diffraction (XRD) spectra were obtained by X’per PRO, PANalytical, using Cu Kα radiation. Ultraviolet−visible (UV−vis) diffuse reflectance spectra were recorded on an UV-2550 spectrometer.
Figure 1. (a) Schematic procedure for the synthesis of Vo-Fe−Pi/VoFe2O3. (b) TEM of Fe2O3 NFs. Inset of (b) shows the selected area electron diffraction pattern. (c−e) HRTEM images of (c) Fe2O3, (d) Fe−Pi/Fe2O3, and (e) Vo-Fe−Pi/Vo-Fe2O3. (f, g) Line scans and (h) TEM EDX elemental mappings of Vo-Fe−Pi/Vo-Fe2O3. (i, j) O 1s and P 2p XPS spectra of Vo-Fe−Pi/Vo-Fe2O3.
the defective crystalline Fe−Pi cocatalyst; meanwhile, oxygen vacancies were introduced in ultrathin Fe2O3 (denoted as VoFe−Pi/Vo-Fe2O3). During the synthesis, highly active Vo-Fe− Pi/Vo-Fe2O3 photoanodes are obtained for PEC water splitting. Scanning electron microscopy (SEM) image shows flakes with 1.5−2.0 μm thickness and 100 nm base grown on iron substrate (Figure S1a). The morphology of Fe−Pi/Fe2O3 is similar to that of the nanoflakes. The side surface of the asprepared sample is smooth and remains after Fe−Pi decoration (Figure S1b). Figure 1b−d shows transmission electron microscopy (TEM) images of Fe2O3 without or with Fe−Pi decoration. The (110) crystal facet can be seen in the crystalline Fe2O3 with a smooth outer surface (Figure 1b,c). However, for Fe−Pi/Fe2O3, an amorphous layer with ∼5 nm thickness is observed on the outer surface (Figure 1d), consisting of iron phosphide/phosphate. The flakes with this overlayer were further etched in Ar-plasma condition. Apparently, this amorphous layer became thinner (2−3 nm) compared to Fe−Pi/Fe2O3 (Figure 1e). To elucidate the composition of this overlayer, TEM line scans, energydispersive X-ray (EDX) elemental mapping, and X-ray photoelectron spectroscopy (XPS) analysis on Vo-Fe−Pi/VoFe2O3 were performed as shown in Figure 1f−j. The findings
3. RESULTS AND DISCUSSION Figure 1a shows a schematic procedure for the synthesis of plasma-induced oxygen vacancies in Fe−Pi/Fe2O3 core−shell sample. First, Fe2O3 nanoflakes were prepared on iron substrates by thermal oxidation method.29,30 The as-formed Fe2O3 were then immersed in 5 mM NaH2PO4 solution for a certain time, and a thin amorphous FePO4 layer was conformally coated on the surface (denoted as Fe−Pi/ Fe2O3). Finally, Ar-plasma treatment was applied to obtain 22273
DOI: 10.1021/acsami.9b03789 ACS Appl. Mater. Interfaces 2019, 11, 22272−22277
Research Article
ACS Applied Materials & Interfaces
(inset of Figure 2b), and the peak becomes slightly weaker, indicating that increasing disorder and oxygen vacancy concentration resulted from Ar-plasma treatment. Indeed, the plasma bombardment removes more oxygen atoms from the interface of the Fe−Pi cocatalyst and Fe2O3. Besides, the plasma treatment did not change the main composition of Fe2O3 but generated more oxygen vacancies, which would influence the PEC activity in Vo-Fe−Pi/Vo-Fe2O3. Fourier transform infrared spectrophotometry (FTIR) measurement was performed to investigate the structure of the overlayer on Fe2O3. As shown in Figure 2c, four phosphate IR bands are observed at 895, 968, 1046, and 1189 cm−1. The band at 1046 cm−1 can be assigned to the symmetric stretching vibration of the P−O−P nonbridging oxygen bond, and the bands at 968 and 1189 cm−1 are attributed to the symmetric and asymmetric stretching vibrations of P−O−P linkages, respectively.35,36 These bands resemble those reported for FePO4.34,37 The ultraviolet−visible (UV−vis) absorption spectra (Figure 2d) shows that the Fe−Pi/Fe2O3 and VoFe−Pi/Vo-Fe2O3 samples exhibit higher light absorption compared to the pristine Fe2O3, in agreement with Delaunay et al.’s study.23 The samples have a absorption edge around 600 nm, which corresponds to the characteristic absorption edge of Fe2O3 reported in the literature.10 The wettability was investigated from the contact angle measurements as shown in Figure S3. The pristine Fe2O3 exhibits a hydrophilic nature with a contact angle of 17°, whereas Vo-Fe−Pi/Vo-Fe2O3 shows better hydrophilic surface properties with a contact angle of 2°. The hydroxyl group at the surface usually dominates the charge transfer across the photoanode/electrolyte interface, as these hydroxyl groups participate in the oxygen evolution reaction. The samples were applied as photoanodes for PEC performances in the dark and light under simulated 1 sun illumination in 1 M KOH electrolyte. The NaH2 PO 4 immersion time and Ar-plasma treatment time were first optimized as shown in Figures S4 and S5 with respect to the best photocurrent. Figure 3a shows the J−V curves for the pristine Fe2O3, Fe−Pi/Fe2O3, and Vo-Fe−Pi/Vo-Fe2O3 photoanodes. Note that Fe−Pi decoration has an effect on the PEC activity. The Fe−Pi/Fe2O3 heterojunction photoanode shows an improved photocurrent from 0.63 mA cm−2 (pristine) to 1.20 mA cm−2 at 1.23 VRHE, accompanied by a negative shift of the onset potential (0.1 VRHE). The enhancement of the photocurrent and the shift of the onset potential are attributed to the thin Fe−Pi as a water oxidation cocatalyst. Correspondingly, the incident photon-to-current efficiency (IPCE) of Fe−Pi/Fe2O3 displays higher values at the photon wavelengths ranging from 400 to 700 nm compared with that of Fe2O3 (Figure S6). A control experiment is conducted by dipping the NaH2PO4 solution in Fe2O3 (denoted as PO43−/ Fe2O3). The photocurrent remains the same as that of the pristine Fe2O3 (Figure S7), representing that PO43− does not serve as an OER cocatalyst. P-doped Fe2O3 was also prepared by annealing PO43−/Fe2O3 in air, and the J−V curve is increased to 1.72 mA cm−2 at 1.23 VRHE (Figure S8). Moreover, Ar-plasma-treated Fe2O3 shows a photocurrent of 1.50 mA cm−2 at 1.23 VRHE (Figure S5). Further Ar-plasma treatment on Fe−Pi/Fe2O3 photoanode leads to a remarkably increased current density, 2.71 mA cm−2 at 1.23 VRHE and 3.5 mA cm−2 at 1.50 VRHE. The IPCE value of Vo-Fe−Pi/VoFe2O3 is up to 53% at 360 nm (Figure S6). The performance improvement of Vo-Fe−Pi/Vo-Fe2O3 is a result of the
confirmed the homogeneous distribution of Fe, P, and O in a single flake (Figure 1f−h). Figure 1i,j shows the P 2p and O 1s XPS spectra of Vo-Fe−Pi/Vo-Fe2O3. The O 1s spectrum (Figure 1i) can be fitted with three individual peaks, corresponding to lattice oxygen (530.0 eV), chemisorbed oxygen (531.2 eV), and the defect sites with a low oxygen coordination (532.7 eV).19,23,31 A shoulder becomes stronger after the formation of the Fe−Pi layer and Ar-plasma treatment (Figure S2). The proper concentration of oxygen vacancy favors the improvement on PEC performance by increasing the carrier density.32 The P 2p peak with a single peak at 133.4 eV belongs to the phosphate groups in pyrophosphate (Figure 1j), in agreement with the TEM observation. These characteristics demonstrate that Ar-plasma treatment introduces oxygen vacancies into the Fe−Pi cocatalyst and the bulk Fe2O3. The structural characterization of the materials was recorded by X-ray diffraction (XRD). The XRD spectra of all samples (Figure 2a) can be indexed into hematite (JCPDS 33-0664)
Figure 2. (a) XRD patterns, (b) Raman spectra, (c) FTIR spectrum, and (d) UV−vis spectra of Fe2O3, Fe−Pi/Fe2O3, and Vo-Fe−Pi/VoFe2O3. Inset of (b) shows the Raman spectra in the range of 180-250 cm-1, and Raman spectrum of Fe-Pi on iron.
and magnetite (JCPDS 19-0629) phases. No other composition was found during the chemical precipitation and plasma treatment. Raman spectroscopy has been employed to study the adsorption on the surface and plasma treatment. The Raman spectrum of Fe2O3 (Figure 2b) exhibits peaks at 220 and 495 cm−1, corresponding to Ag, and peaks at 240, 402, and 609 cm−1 for Eg. No additional peaks associated to FePO4 is observed; however, a negative shift in peaks for Fe−Pi/Fe2O3 can be seen (inset of Figure 2b), in agreement with reports of Yu et al.18 and Alshareef et al.33 This would be influenced by an induced strain between surface longer Fe−P bonds and Fe− O, in which the lengthening of the interionic distance is related to a decrease of the force constants between pairs of ions, resulting in a negative shift.33 Additionally, we use the same method for Fe−Pi decoration on the iron substrate, and the Raman spectrum is shown in the inset of Figure 2b. The prominent stretching modes of PO43− structural units are shown at 1000−1100 cm−1, and the bending modes of PO43− are detected below 600 cm−1, confirming the formation of Fe− Pi.34 As Fe−Pi/Fe2O3 was treated in Ar-plasma condition, a further negative shift in the Raman spectrum was observed 22274
DOI: 10.1021/acsami.9b03789 ACS Appl. Mater. Interfaces 2019, 11, 22272−22277
Research Article
ACS Applied Materials & Interfaces
ascribed to the reduced surface recombination. Furthermore, the resistance of the Vo-Fe−Pi/Vo-Fe2O3 photoanode (914 Ω) is lower than that of Fe−Pi/Fe2O3 (1105 Ω), which suggests that the Ar-plasma treatment substantially increases the charge transfer between the electrode and the electrolyte by increasing the concentration of oxygen vacancies. Note that Vo-Fe−Pi/Vo-Fe2O3 was formed by a simple precipitation and plasma treatment and did not require complex fabrication approaches, but it still shows a high PEC performance for water splitting. Then, the formed Fe−Pi cocatalyst can catalyze OER efficiently for water splitting. To study the activity of photoanodes for water oxidation, and the surface states related to the cocatalyst and the oxygen vacancies, charge and discharge processes were conducted. Fe3+/Fe2+ redox couples form the surface states by accepting electrons or holes during the charge/discharge process.18 From the plots (Figure 4a), an anodic peak between 0.8 and 1.1 V in
Figure 3. (a, b) Linear sweep voltammogram curves in (a) light and (b) dark, (c) applied bias photon-to-current efficiencies, (d) current− time curves, (e) Mott−Schottky plots, and (f) EIS spectra of Fe2O3, Fe−Pi/Fe2O3, and Vo-Fe−Pi/Vo-Fe2O3.
inherently increased amount of oxygen vacancies in the Fe−Pi layer and Fe2O3 for water oxidation. However, the onset potential of Vo-Fe−Pi/Vo-Fe2O3 exhibits a positive shift relative to that of Fe−Pi/Fe2O3, which is ascribed to the increasing surface defects at the interface. Additionally, the Fe−Pi/Fe2O3 photoanode shows an obviously negative shift in the dark currents compared to that of Fe2O3 (Figure 3b), resembling that of the water oxidation cocatalyst Co-Pi.38−40 This implies that Fe−Pi acts as water oxidation catalyst. With further Ar-plasma treatment, the water oxidation onset potential of Vo-Fe−Pi/Vo-Fe2O3 in the dark curve is similar to that of Fe−Pi/Fe2O3. The calculated applied bias photonto-current efficiency is 0.47% at 0.8 VRHE (Figure 3c). The Arplasma activation can introduce abundant defects (dislocations), which would provide more active sites at the interfaces for PEC activities. Besides, the transient current density is examined and shown in Figures 3d and S9. The value of isteady/ iinitial (is/ii) increases from 0.84 to 0.97 from Fe2O3 to Vo-Fe− Pi/Vo-Fe2O3 photoanodes, indicating the reduced recombination of electron−hole during the surface treatment. To further examine the oxygen vacancies upon Ar-plasma treatment, the Mott−Schottky measurements were performed (Figure 3e). From the results, we can conclude that Fe−Pi/ Fe2O3 shows similar curve as that of the pristine Fe2O3, whereas Vo-Fe−Pi/Vo-Fe2O3 exhibits the smallest slope, which implies that the oxygen vacancies enhance the electric conductivity. To illustrate the electrochemical properties of the effect of Fe−Pi and Ar-plasma treatment and the chargetransfer process at the surface of electrode/electrolyte, Figure 3f shows the electrochemical impedance spectroscopy (EIS) results. Significantly, the resistances of Fe−Pi/Fe2O3 (1105 Ω) is lower than that of Fe2O3 (1570 Ω), indicating that the superior charge transfer from Fe−Pi/Fe2O3 to the electrolyte is
Figure 4. (a, b) Cyclic voltammograms in the range of 0.4−1.6 VRHE and 1.27−1.37 VRHE for Fe2O3, Fe−Pi/Fe2O3, and Vo-Fe−Pi/VoFe2O. (c) Electrochemical surface areas of corresponding electrodes.
the cyclic voltammogram is detected for Fe2O3, corresponding to the oxidation of surface Fe2+ in the oxygen-abundant regions. The reduction peak for Fe−Pi/Fe2O3 is substantially larger than that for Fe2O3, which is an evidence of the number of active surface states accessible as the Fe−Pi cocatalyst is decorated.41,42 With further Ar-plasma treatment, the reduction peak of Vo-Fe−Pi/Vo-Fe2O3 becomes slightly higher than that of Fe−Pi/Fe2O3. Besides, a second set of cyclic voltammograms, evaluating the capacitive nature of the electrode, was recorded from 1.27 to 1.37 VRHE (Figure 4b). As expected, Fe−Pi/Fe2O3 and Vo-Fe−Pi/Vo-Fe2O3 electro22275
DOI: 10.1021/acsami.9b03789 ACS Appl. Mater. Interfaces 2019, 11, 22272−22277
Research Article
ACS Applied Materials & Interfaces des have substantially higher current densities than that of Fe2O3, which means a higher surface area after Fe−Pi deposition and Ar-plasma treatment. Vo-Fe−Pi/Vo-Fe2O3 exhibits a higher current density compared to Fe−Pi/Fe2O3, implying an enhancement in the pseudocapacitive performance resulting from plasma activation. The Cdl value increases more than one time after Fe−Pi deposition (Figures 4c and S10), indicating a higher augmentation of Aechem and exposure of more active sites to the electrolyte.31 During the simple precipitation process, the long immersion time in the NaH2PO4 electrolyte on Fe2O3 causes the dissolution of Fe3+ and Fe2+ on the hematite surface. Meanwhile, a long pretreatment time damages the flake’s surface, resulting in a disordered crystalline surface. The dissolved iron ions were examined by inductively coupled plasma mass spectrometry analysis. Five milliliters of NaH2PO4 aqueous solution contained ca. 0.386 mg L−1 iron element after 15 h of treatment, which confirms the dissolution of hematite and the formation of Fe−Pi. Furthermore, the dissolved Fe3+/ Fe2+ ions indicate the decrease in dangling bonds and oxygen vacancies, or the passivation of the surface states. The amorphous Fe−Pi with disordered structure has a low-energy level d band center and less Gibbs free energy to benefit oxygen evolution, improving the electrocatalytic activity.42 Additionally, we also prepared Fe2O3 samples with other morphologies, e.g., nanocoral. It is definitely sure that Fe−Pi has an OER activity on this structure (Figure S11), whereas for this special structure, FeOOH acted as a cocatalyst on Fe2O3 nanocorals, leading to a decay in the photocurrent (Figure S12), except for the onset potential in dark and light. For plasma treatment, different structures have shown different experimental phenomena, indicating that this treatment depends strongly on the structure.43,44 For instance, for the nanocoral, this treatment has no more effect on it. Fe2O3 flakes with ultrathin structures are beneficial for the activation of oxygen vacancies, further improving the PEC performance.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Lei Wang: 0000-0001-7449-2763 Xianhu Liu: 0000-0002-4975-3586 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (51802320), start-up fundings from Inner Mongolia University and Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS) instrument function development technology innovation project, and the opening project of Key Laboratory of Materials Processing and Mold.
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REFERENCES
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4. CONCLUSIONS In summary, we show that a simple chemical precipitation (Fe−Pi deposition), followed by Ar-plasma treatment on Fe2O3 could simultaneously improve the photocurrent density and the interfacial property of cocatalyst/Fe2O3 photoanodes. A surface phosphate treatment accelerates the hole transfer from the bulk to the surface. Besides, creating defects (oxygen vacancies) on the cocatalysts by an Ar-plasma treatment can significantly increase the reactivity of the active sites, resulting in a remarkably enhanced OER activity and PEC performance. The Vo-Fe−Pi/Vo-Fe2O3 photoanode has a current density of 2.71 mA cm−2 at 1.23 VRHE and 3.5 mA cm−2 at 1.50 VRHE in 1 M KOH under AM1.5 simulated solar light condition. This work points to a new opportunity for optimizing the performance of hematite and achieving a highly efficient PEC system without a complicated approach or complex reaction.
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current−time curves; CV curves of Fe2O3; NCs samples; and stability of Vo-Fe−Pi (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03789. SEM images; XPS spectra; contact angles; LSV curves of Fe2O3; LSV curves of Vo-Fe2O3; IPCE values of Fe2O3; 22276
DOI: 10.1021/acsami.9b03789 ACS Appl. Mater. Interfaces 2019, 11, 22272−22277
Research Article
ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.9b03789 ACS Appl. Mater. Interfaces 2019, 11, 22272−22277
