Manipulation of Charge Transfer in FeP@Fe2O3 CoreâShell

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Manipulation of Charge Transfer in FeP@Fe2O3 CoreShell Photoanode by Directed Built-In Electric Field Lin Yang, Yuli Xiong, Li Li, Yibin Yang, Di Gao, Lu Liu, Hongmei Dong, Peng Xiao, and Yunhuai Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00756 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Energy Materials

Manipulation of Charge Transfer in FeP@Fe2O3 Core-Shell Photoanode by Directed Built-In Electric Field

Lin Yang,† Yuli Xiong,† Li Li,† Yibin Yang,† Di Gao,‡ Lu Liu,‡ Hongmei Dong,‡ Peng Xiao,*†‡ and Yunhuai Zhang*‡

†

Chongqing Key Laboratory of Soft Condensed Matter Physics and Smart Materials, College of Physics, Chongqing University, Chongqing 400030, China

Email: (P. Xiao) [email protected] ‡

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing

400030, China Email: (Y. Zhang) [email protected]

Notes The authors declare no competing financial interest.

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Hematite (Fe2O3) can be suitable used in solar energy conversion system, but the short charge diffusion lengths limit its applications. Here, we report the studies of charge transfer ability with 40 nm Fe2O3 nanorod decorated by 5 nm iron phosphide (FeP) core-shell structure. By selecting the optimized time of phosphorization (20 min), the photocurrent of FeP@Fe2O3-20 photoanode reached 0.86 mA/cm2, enhanced by 4.10 folds compared with pristine Fe2O3 (0.21 mA/cm2) for water oxidation. Further, the charge transport time reduced by 30% due to the FeP shell served as the hole transport layer. Compared with Fe2O3, the FeP@Fe2O3 has a higher Fermi level, which guides the electron’s transfer from FeP to Fe2O3 to create a space charge layer. The charge balance induces an upward bending of band structure at the FeP and Fe2O3 interface and accelerates the separation of photogenerated electron-holes ascribe to the built-in electric field at the interface. Our studies provide a detailed understanding of carrier dynamics in the core-shell structure, demonstrating a new route to explore high efficiency approaches for solar harvesting.

KEYWORDS: FeP@Fe2O3, charge separation, oxidation kinetics, built-in electric field, surface potential, photoelectrochemical

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1. INTRODUCTION Photoelectrochemical technology (PEC) is regarded as a perfect route to convert solar energy to chemical energy.1-3 In general, the overall PEC reaction consists of three decisive processes:4,5 i) the photon-absorption by a semiconductor to induce the electron and hole, ii) charge separation and transport, and iii) redox reaction with holes and electrons. However, the process of these steps have to encounter numerous obstacles.6 To deal with the low efficiency of carriers’ migration and photo absorption, numerous narrow bandgap semiconductors were developed, such as BiVO4 and BiFeO3.5 Hematite (Fe2O3), a promising semiconductor material, exhibits fascinating photo absorption property due to the suitable bandgap (～2.1 eV).7,8 Besides, the valence band of Fe2O3 is sufficiently positive (～2.4 V vs. RHE) and providing sufficient overpotential for holes to hold a photooxidation kinetics, also has an excellent chemical stability in neutral and alkaline electrolytes.9 Theoretically, the maximum photocurrent of Fe2O3 photoanode under AM 1.5G illumination is 12.6 mA/cm2 for water oxidation.10 However, the practical photocurrent is much lower than the theoretical value ascribed to the limited electrical conductivity, slow charge separation, sluggish charge transfer, and short hole diffusion lengths (2-4 nm).11-13 During the past years, many works focused on design and develop Fe2O3 based photoanodes for PEC reactions. Specially, Gong et al. fabricated a TiO2 interlayer on Fe2O3,14 which could enhance the photocurrent by decrease the FTO/Fe2O3 interfacial charge recombination. The multi-layered structures based photoelectrodes could be regarded as one of the promising candidates for efficient solar conversion devices due 2



to the decreased defect state at interface.15 In other words, a strongly localized electric field may be exist at the interfacial of different materials due to charge balance, which could bend the band structure and accelerate charge migration. Park et al. fabricated a porous Fe2O3 structure doped by Sn and coated with a Nb2O5 passivation layer (∼2 nm),16 the enhanced donor density with 20 folds and dramatically increased conductivity were observed, bringing to an improved charge transfer ability. However, the photo to current conversion efficiency was still very low. Thus, developing new structures to enhance the charge separation and oxidation kinetics efficiencies of Fe2O3 based photoanodes are remaining a challenge. Recently, much efforts displayed that the transition metal phosphides possessed a fast charge transfer ability, especially for iron phosphide (FeP).17-19 Unlike transition metal chalcogenides,20 FeP is difficult to form layered structures. Hence, FeP potentially allows more active corners on the crystallite surface, making it distinctively advantageous for water splitting.21,22 Unfortunately, there are no precious investigations for Fe2O3 decorated by FeP nanostructure with a precisely controlled thickness that served as a carrier transport layer for PEC application. So, if we can decorate the Fe2O3 with a nano-sized FeP layer, the efficiencies of charge separation and oxidation kinetics are likely to be enhanced. Herein, we developed an approach to synthesis the FeP@Fe2O3 core-shell photoanode, which exhibits an enhanced photocurrent of 4.10 folds compared with pristine Fe2O3. This novelty structure consists a 40 nm core (Fe2O3) and a 5 nm shell (FeP). Through evaluating the band structure of Fe2O3 and FeP, the low surface 3


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potential of FeP suggest a lower work function than Fe2O3, indicating the Fermi level of FeP is higher than Fe2O3. The different Fermi level of materials induces the electron’s flow from FeP to Fe2O3, which can create a stable built-in electric field to accelerate the holes’ migration. Based on the experimental results, the strategy of phosphorization engineering for Fe2O3 to form a core-shell structure offers a highly charge separation and oxidation kinetics in solar energy conversion.

2. EXPERIMENTAL SECTION 2.1 Synthesis of Fe2O3 Nanorod Arrays. Synthesis of Fe2O3 nanorod arrays on the FTO substrate includes two steps: preparation of FeOOH nanorod arrays and transfers it to Fe2O3 by subsequent heat process. Briefly, 0.1 M FeCl3 and 1 M NaNO3 solution was prepared as the precursor. The concentrated HCl was added into the solution to adjust the pH to 1.25 and the reseda solution was obtained. The FTO glass was immersed in this solution (20 mL) with a certain inclination (about 60° and the conductive surface is faced down) and put it into a Teflon-lined stainless steel autoclave. The autoclave was moved into an oven and maintained at 100 °C for 6 h. Then, a slight yellow FeOOH was growing onto the FTO substrate vertically. Lastly, the sample was cleaned by deionized water and annealing at 700 °C for 20 min with 2 °C/min to form Fe2O3 nanorod arrays. 2.2 Synthesis of FeP@Fe2O3 Core-Shell Photoanode. Synthesis of FeP@Fe2O3 involves only one step by using a tube furnace. Briefly, 0.5 g NaH2PO2 was placed in the inlet, while Fe2O3 was placed in a back position (about 10 cm). This system was 4



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annealing at 300 °C for tens of minutes with 5 °C/min, and the FeP@Fe2O3 core-shell structure photoanode was obtained. In order to investigate various phosphorization depths for photoanodes, we chose different annealing time (1, 5, 10, 20, 30, 50, and 80 min), and named these photoanodes are FeP@Fe2O3-1, FeP@Fe2O3-5, FeP@Fe2O3-10,

FeP@Fe2O3-20,

FeP@Fe2O3-30,

FeP@Fe2O3-50,

and

FeP@Fe2O3-80, respectively. When the phosphorization time is reached, we use a pump to vacuum the quartz tube, and insure to shut off the phosphorization process.

3. RESULTS AND DISCUSSION The fabrication process of FeP packaged Fe2O3 core-shell nanostructure (denoted as FeP@Fe2O3) is illustrated in Figure 1a. Firstly, the FeOOH nanorod (NR) arrays were grown on the FTO substrate vertically by a conventional hydrothermal reaction (at 100 °C for 6 h).14 Secondly, the FeOOH film was transformed into Fe2O3 after thermal treatment in air at 700 °C for 20 min. Lastly, the as-prepared Fe2O3 was placed in a tube furnace to process a phosphorization with a second thermal treatment at 300 °C in N2 flow for various time (1, 5, 10, 20, 30, 50, and 80 min), and the FeP packaged Fe2O3 core-shell nanostructure was successfully obtained. The morphology of the as-prepared FeOOH was investigated by scanning electron microscopy (SEM), demonstrating the FeOOH were homogeneous deposited on the FTO substrate, and the diameter of FeOOH NR arrays is about 40 nm (Figure S1). Further, X-ray diffraction (XRD) pattern indicate that the crystallized FeOOH powder is obtained, which matched well with the standard card of 81-0464 (Figure 5


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S2). Figure 1b shows the SEM image of Fe2O3, indicating the average sizes of the Fe2O3 NRs are about 40 nm and grow vertically on the FTO substrate. After phosphorization, the morphology of FeP@Fe2O3 present a similar structure with Fe2O3 (Figure 1c-e). However, after phosphorization for 80 min, the surface of FeP@Fe2O3-80 begins to congeal and the NR arrays are collapsed. Hence, longer time of phosphorization may destroy the FeP@Fe2O3 core-shell structure, and decreases the specific surface area for PEC reactions. We also further analyze the bulk structures of these semiconductors by XRD, which conducted using powder samples scraped from the FTO substrate (Figure S3a). The XRD patterns reveal that the obtained Fe2O3 is the α-Fe2O3 phase (87-1166),23 the peaks of the diffraction angles at 2θ of 24.15° and 33.16° could be assigned to the (012) and (104) plane for typical Fe2O3. Due to the small quantity of FeP shell, the XRD data of FeP@Fe2O3 core-shell structure demonstrate a similar peaks position of Fe2O3, which is corresponding to other reports.24,25 To identify the FeP out layer, we evaluate the XRD data for FeP powder. The pure phase FeP sample was synthesized by phosphorizing Fe2O3 in 1 g NaH2PO2 as the phosphorus source and treated for 5 h. Adding large amount of NaH2PO2 and lengthening the phosphorization time can ensure the Fe2O3 are completely transformed into FeP, and the XRD data indicate that the pure FeP was obtained (Figure S3b).26 In addition, we use transmission electron microscopy (TEM) to investigate the structure of Fe2O3 and FeP@Fe2O3 photoanodes. The lattice fringes was observed by HRTEM, showing the interplanar crystal spacing are 0.27 and 0.37 nm, corresponding the exposed crystal planes of (012) and (104) for Fe2O3 and 6



consistent with bulk XRD data (Figure 2b). These results prove that the as-prepared sample is the well crystallized Fe2O3 NR. Additionally, the TEM image of FeP@Fe2O3-20 demonstrate that the Fe2O3 NR is surrounded by FeP and forming a core-shell structure (Figure 2c). After phosphorization for 20 min, we can obtain a thin FeP shell, the thickness is about 5 nm. Wang et al. demonstrated that, for a surface covering material,24 when the thickness is lower than 10 nm, it will not affect the photo-absorption capacity for the inner semiconductors. Furthermore, the energy dispersive X-ray (EDX) spectrum for FeP@Fe2O3-20 reveals that the atomic percentage of O, P, and Fe is 64.84%, 1.72%, and 33.43%, respectively (Figure S4). The highly proportion of O can be ascribed to the surface oxidation when the sample exposed to air, and forming the PO43- species.26,27 What’s more, EDX elemental mapping images confirm the homogeneous distribution of O, Fe, and P elements throughout the overall core-shell structure (Figure 2d). Further evidences for surface information of Fe2O3 and FeP@Fe2O3 samples were obtained by X-ray photoelectron spectroscopy (XPS) analysis. The main carbon signal at 284.8 eV from the CO2 was used as the reference to calibrate all XPS data. As for the high resolution of Fe 2p (Figure S5a), two peaks at 712.1 and 725.7 eV can be ascribed to the 2p3/2 and 2p1/2 of Fe3+, which agree well with literature reports for Fe2O3.28,29 Besides, the P 2p spectrum indicates two peaks at 129.3 and 130.2 eV (Figure S5b), corresponding to the binding energy of P 2p1/2 and P 2p3/2.28 This doublet (129.3 and 130.2 eV) can be regarded as the P bonding to Fe. However, the highly binding energy at 134.4 eV in P 2p could be ascribe to the PO43- species when 7


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the FeP was exposed to air.27 Obviously, the P 2p binding energy of 129.3 eV shows negative shift from elemental P (130.2 eV), revealing Fe and P in FeP carry partial positive and negative charge. In short, the XPS data demonstrate the occurrence of electron transfer from Fe to P in FeP, which is matched well with previous observations for CoP, Cu3P, and MoP.30-32 In addition, we conducted optical properties of powder samples to investigate their light absorption abilities and the width of bandgap. The ultraviolet-visible absorption spectra indicate the strong UV and visible light absorption value for Fe2O3 and FeP@Fe2O3 structure (Figure S6), demonstrating the typically photoelectric characteristics (the onset absorption is about 600 nm) and agree with previous reports.14 What’s more, the Tauc plots indicate an indirect gap of these samples, revealing the bandgap of these semiconductors is about 2.08 eV and matched well with other researches.14 To deeply explore how the FeP shell affect the activity of photoanodes in water splitting system, PEC properties under AM 1.5G were acquired by front-side illumination. In order to obtain the photo-response of various photoanodes, typical current-potential (J-V) curves were collected by a PEC cell in neutral electrolyte (pH 7.0). With the PEC reaction, photoanode is used for water oxidation (2H2O + 4h+ → O2 + 4H+), while the counter electrode (Pt wire) is used for water reduction (2H+ + 2e→ H2).9 To confirm the properties of FTO substrate is almost unchanged by annealing at 700 °C, cyclic voltammetry was performed and indicating that the cyclic voltammetry curves were remain unchanged in a large potential range (0.5-1.8 V vs. RHE) (Figure S7a). What’s more, the roughness of the surface of photoanode can 8



intuitively reflect the react surface area, which can be conveniently obtained by chronoamperograms. Figure S7b suggests the real surface areas of Fe2O3, FeP@Fe2O3-5, FeP@Fe2O3-20, and FeP@Fe2O3-80 are 9.3, 9.2, 9.2, and 9.1 cm2, respectively. Obviously, these photoanodes have a comparable real surface area for PEC reactivity. In order to identify the effect of oxygen defects (maybe produced during the treatment at 300 oC in N2), the photocurrent of Fe2O3 with/without treated in N2 at 300 oC for 20 min were compared (Figure S8a). The results indicate that there have no clearly changes in the J-V curves, suggesting we need not take the oxygen defects into consideration for our experiment. Then, the J-V curves with water oxidation of various photoanodes with/without illumination were evaluated. We conducted the analysis for 5, 20, and 80 min phosphorization samples (Figure 3a), the highest photocurrent is obtained by phosphorization for 20 min (Figure S8b). Owing to the sluggish oxidation kinetics of un-modified photoanode, the photogenerated electron-hole pairs are easily recombination on the surface of Fe2O3, displaying a weak photocurrent (0.21 mA/cm2 at 1.23 V vs. RHE). After decorated by FeP shell, the core-shell photoanode (FeP@Fe2O3-20) demonstrate an increased photocurrent responses (0.86 mA/cm2 at 1.23 V vs. RHE), demonstrating an enhancement of 4.10 folds compared with pristine Fe2O3. After phosphorization for 80 min, the photocurrent decrease to 0.55 mA/cm2 at 1.23 V vs. RHE, which may be ascribe to the agglomerate at the surface of the photoanode (see SEM data).33 In addition, the onset potential of all samples are comparable (about 0.69 V vs. RHE), suggesting that the FeP has little impact on the onset photocurrent with water oxidation. This 9


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dramatically improvement in photocurrents is ascribed to the FeP shell which could be served as a hole transport layer. Additionally, the greatly improved percentage of photocurrents by this core-shell photoanode is much higher than other core-shell structure (ZnO/TiO2 and SrTiO3/TiO2).24,34 To elucidate the mechanism of charge transfer of various photoanodes, flatband potentials were measured in 0.5 M phosphate buffer. Figure S10 presents the Mott-Schottky plots at different frequencies, which show a quasilinear behavior and indicate the as-prepared samples are n-type semiconductor.10 The flatband potentials of Fe2O3 and FeP@Fe2O3-20 are about 0.60 and 0.52 V vs. RHE. The negative shift of flatband potential for FeP@Fe2O3 suggests an enhanced charge migrate kinetics for the core-shell structure. Further, the slops of these Mott-Schottky plots reflect that the coated FeP shell could increase the carrier concentration dramatically, also confirm the core-shell structure can simultaneously promote the charge separation and oxidation kinetics again. Compared with the previous works, our results displayed a highly enhanced photocurrent behavior (Table S1). As expected, the similar phenomenon of the improved photocurrents has also been observed for sulfite oxidation by adding 0.2 M Na2SO3 in the electrolyte (Figure 3b). The sulfite oxidation result indicates that the photocurrent of FeP@Fe2O3-20 is 1.70 mA/cm2 at 1.23 V vs. RHE and higher than pristine Fe2O3 (0.79 mA/cm2). Furthermore, the onset potentials of Fe2O3 and FeP@Fe2O3-20 are 0.44 and 0.42 V vs. RHE. The lower onset potential for photocurrent also means enhanced charge migrate kinetics. It is worth noting that the PEC performances of all photoanodes can be 10



ranked with the following order: pristine Fe2O3 < FeP@Fe2O3-80 < FeP@Fe2O3-5 < FeP@Fe2O3-20. Nevertheless, to quantify the FeP effect on the charge separation and transfer, the efficiency of charge separation (ηsep) and oxidation kinetics (ηox) could be determined independently, which can be individually calculated as ηsep ≈ Jsulfite / Jabs and ηox ≈ Jwater / Jsulfite.35,36 These results displayed that ηsep of FeP@Fe2O3-20 is 24.8% at 1.23 V vs. RHE, which is higher than pristine Fe2O3 (11.6%) and demonstrating an improved charge separation (Figure 3c). Specifically, ηox is improved from 26.6% for pristine Fe2O3 to 56.6% for FeP@Fe2O3-20 at 1.23 V vs. RHE (Figure 3d), indicating a great advantage for oxidation kinetics due to the FeP shell. These results suggest that the FeP decorated Fe2O3 core-shell structure is beneficial for charge separation and oxidation kinetics, which could be ascribed with the fast interface charge transfer ability. To gain more insight into the mechanism of the photocurrent enhancement, electrochemical impedance spectroscopy (EIS) measurements were performed to discuss the charge transfer behavior in the electrode/electrolyte interface with water oxidation.37 Obviously, the pristine Fe2O3 demonstrate a larger arc radius under illumination compared with the FeP decorated photoanodes (Figure 4a). As for the Nyquist plots, the arcs usually represent the intrinsic resistance of the interface of photoelectrode and electrolyte. Thus, the smaller arc radius displays enhanced interfacial charge transfer ability for photogenerated carriers in this core-shell structure. Besides, to elucidate the electron’s transfer speed in photoanode, we conducted the relation of electrons transport time (τ) and power density (the incident 11


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light wavelength is 365 nm) at 1.23 V vs. RHE for various samples with water oxidation. The electron’s transport time is the average time that the electrons transport from the near surface of photoanode to the FTO substrate, displaying the electron transport properties in the bulk of photoanode. These results are obtained by the controlled intensity modulated photocurrent spectrometer (CIMPS) tests (Figure S11).38,39 Figure 4b plot the different charge transport time (τ) of photoanodes, the τ values for Fe2O3 and FeP@Fe2O3-20 are 1.25 and 0.88 ms under 300 W/m2 illumination during the water oxidation process. Clearly, the photogenerated electrons in pristine Fe2O3 have a longer transport time compared with core-shell photoanode, suggesting lower charge collection efficiency. The essential distinction of the charge transport time is definite correspond with the oxidation kinetics for photoanodes under illumination, showing the ability of charge transfer predominating overall water oxidation. Additionally, these results are consistent with their photocurrent curves for water oxidation. As discussed above, to further understand the mechanism of simultaneously enhanced charge separation and oxidation kinetics, a kelvin probe force microscopy (KPFM) was used to study the surface potentials (Vsurface) of Fe2O3 and [email protected],41 Figure 5a and b show the KPFM and topology images of the as-prepared samples, indicating the similar structure of the two type photoanodes on FTO substrate. Interestingly, the surface potential images of various samples reveal the Vsurface of Fe2O3 and FeP@Fe2O3-20 are 125 and 78 mV (Figure 5c) respectively. For a semiconductor surface, the measured Vsurface is related to the work function of 12



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the material, as the following equation:42 Φsample = Φreference + e·Vsurface

(1)

where Φsample and Φreference are the work function of the material and the probe respectively. So, we can obtain the Fermi level (EF) of a semiconductor according to the relationship of work function and vacuum energy level of electron (W), Φsample = W – EF

(2)

However, as for a core-shell structure, the Vsurface is related with the surface energy of a material, which differs from the bulk work function ascribe to the formed space charge region near the interface. Therefore, the Vsurface mainly reflect the properties of the shell material. Hence, we can make a conclusion that the decreased Vsurface indicate a lower work function and higher EF, which can induce a built-in electric field (Ein) at the interface of the semiconductors,43 and the direction of the field vector is towards the materials with highly EF (from Fe2O3 to FeP), the enhanced charge migration system is demonstrated in Figure 6a. When visible light radiates on the photoanode, the photogenerated electrons and holes are generated in Fe2O3 core. The FeP shell serves as the holes transfer layer and reduces the recombination of electron-hole pairs due to the Ein at the interface of FeP and Fe2O3, boosting the separation of electron-holes. Subsequently, the photoinduced electrons will transfer from the near surface of the photoanode to FTO substrate. Meanwhile, FeP also serves as the water oxidation points, draws electrons from the water molecules and produces oxygen. Furthermore, Figure 6b presents the energy band structure of FeP@Fe2O3 for PEC reaction. The Fermi level EF of FeP is higher than that of Fe2O3 according to the 13


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work function analysis, leading to an upward band bending structure at the interface. Accordingly, this band structure can not only increase the charge separation but also stimulate the transfer of holes from Fe2O3 to FeP layer efficiently, thus accelerating the oxidation kinetics. In short, the highly PEC performance of FeP@Fe2O3 core-shell structure can be attributed to the following major factors: i) the vertically oriented Fe2O3 NR arrays ensure enough real react surface, which allows easy diffusion of electrolyte into the bulk photoelectrode; ii) the FeP possesses a lower Vsurface, which can induce an outside directed Ein to accelerate charge migration. In addition, the incident photon-to-electron conversion efficiency (IPCE) reveals that the spectra are integrated and accordance the photocurrent results (Figure S12).14 For all photoanodes, the photoresponse was observed from 350 to 650 nm, which is consistent with their UV-vis absorbance spectra. Compared with Fe2O3, the core-shell photoanode exhibits an enhanced IPCE over the entire absorption region, FeP@Fe2O3-20 presents a higher IPCE (40.8%) compared with pristine Fe2O3 (8.4%) at 500 nm. The other important consideration for photoanodes is their stability over prolonged illumination.9 Figure S13 shows the stability of various samples at 1.23 V vs. RHE in neutral solution (0.5 M phosphate buffer, pH 7.0). The FeP@Fe2O3-20 sample suggests an obvious stabilization in photocurrent for water oxidation (∼97%), emphasizing a better stability. Nevertheless, the Fe2O3 retains ∼93% of its initial photocurrent over the longtime reaction, revealing the sluggish charge transfer would hinder the carrier’s transfer and lead to a serious photocorrosion. In general, we have provided a successful hydrothermal, annealing, and phosphorization process to 14



synthesize FeP decorated Fe2O3 photoanodes, which demonstrate an excellent charge separation and oxidation kinetics characteristics (Table 1).

4. CONCLUSION In summary, we have demonstrated a vertically aligned hydrothermal-grow FeOOH NRs on the FTO substrate, and phosphorization Fe2O3 to form the FeP@Fe2O3 core-shell structure. After phosphorization Fe2O3 for 20 min, an enhanced photocurrent of 0.86 mA/cm2 with water oxidation (at 1.23 V vs. RHE) was obtained on the sample FeP@Fe2O3-20, nearly 4.10 folds enhancement compared with pristine Fe2O3 (0.21 mA/cm2) under the AM 1.5G illumination. Specifically, the FeP@Fe2O3-20 sample demonstrated the charge separation and oxidation kinetics efficiency about 24.8% and 50.6% respectively, which were also higher than that of Fe2O3 (11.6% and 26.6%) at 1.23 vs. RHE. The electron transport time was reduced ~30% according to CIMPS analysis. The significant enhancement photocurrent for FeP@Fe2O3 can be ascribed to the lower Vsurface of FeP which can induce a built-in electric field to accelerate charge migration. This finding reported here provide a novel route to design and fabricate the Fe2O3 based systems by phosphorization engineering, which have a potential to further enhance the charge separation and oxidation kinetics for solar energy conversion system.

ASSOCIATED CONTENT Supporting Information Available: Materials, Characterization, Photoelectrochemical 15


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Test, and Calculations for the efficiencies of charge separation and oxidation kinetics, and additional data analysis

ACKNOWLEDGMENTS This work was financially supported by the Graduate Research and Innovation Foundation of Chongqing, China (Grant No. CYS18050) and the Fundamental Research Funds for the Central Universities (Grant No. 106112015CDJZR305501).

REFERENCES (1) Yan, Y.; Crisp, R. W.; Gu, J.; Chernomordik, B. D.; Pach, G. F.; Marshall, A. R.; Turner, J. A.; Beard, M. C. Multiple Exciton Generation for Photoelectrochemical Hydrogen Evolution Reactions with Quantum Yields Exceeding 100%. Nat. Energy 2017, 2, 17052. (2) Pham, T. A.; Ping, Y.; Galli, G. Modelling Heterogeneous Interfaces for Solar Water Splitting. Nat. Mater. 2017, 16, 401-408. (3) Nellist, M. R.; Laskowski, F. A.; Lin, F.; Mills, T. J.; Boettcher, S. W. Semiconductor-Electrocatalyst Interfaces: Theory, Experiment, and Applications in Photoelectrochemical Water Splitting. Acc. Chem. Res. 2016, 49, 733-740. (4) Gao, C.; Wang, J.; Xu, H.; Xiong, Y. Coordination Chemistry in the Design of Heterogeneous Photocatalysts. Chem. Soc. Rev. 2017, 46, 2799-2823. (5) Park, Y.; McDonald, K. J.; Choi, K. S. Progress in Bismuth Vanadate Photoanodes for Use in Solar Water Oxidation. Chem. Soc. Rev. 2013, 42, 2321-2337. 16



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(6) Chen, S. Y.; Yang, J. S.; Wu, J. J. Three-Dimensional Undoped Crystalline SnO2 Nanodendrite

Arrays

Enable

Efficient

Charge

Separation

in

BiVO4/SnO2

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Page 24 of 31

Graphical abstract

V vs. RHE

5 nm FeP

hv

0 1

CB

EF

O2

H2O/O2

H2O FeP

2

5 nm FeP

VB

20 nm

3

Fe2O3

FTO

23


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(a) N2 flow at 300 °C

700 °C for 20 min in air

NaH2PO2

FeOOH/FTO

(b)

FeP@Fe2O3/FTO

Fe2O3/FTO

(c)

(d)

200 nm

200 nm

(e)

200 nm

200 nm

Figure 1 (a) Schematic illustration of the synthesis processes of FeP@Fe2O3 photoanodes using hydrothermal method followed by annealing treatment and phosphorization, (b)-(e) are the SEM images of Fe2O3, FeP@Fe2O3-5, FeP@Fe2O3-20, and FeP@Fe2O3-80, respectively.

24



(a)

Page 26 of 31

(b)

0.37 nm

0.27 nm

20 nm (c)

2 nm O-K

(d) 5 nm FeP 50 nm Fe-K 5 nm FeP

P-K

20 nm

Figure 2 (a) TEM and (b) HRTEM images of Fe2O3, (c) and (d) are the TEM image and EDX elemental mapping of O, Fe, and P for FeP@Fe2O3-20.

25


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Figure 3 PEC performances of various photoanodes, (a) and (b) are the J-V curves for water and sulfite oxidation, while (c) and (d) are the efficiencies of charge separation and oxidation kinetics.

26



Figure 4 (a) EIS Nyquist plots of synthesized photoanodes with water oxidation at the open circuit potential and (b) the electrons’ transport times (τ) at different light intensities (λ = 365 nm) for various photoanodes with water oxidation at 1.23 V vs. RHE.

27


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(a)

(b) 500 nm

500 nm

200 nm

200 nm

(c)

Figure 5 KPFM and topology images (insert figure) of (a) Fe2O3 and (b) FeP@Fe2O3-20, (c) is the surface potential result which detected by the red arrow.

28



(a)

+

Bias

Page 30 of 31

(b) V vs. RHE

-

hv

0

CB

hv H2

Ein

1

EF

O2

H2O/O2

O2

H

+

H2O

VB

3 Fe2O3

FTO

FeP

H2O FeP

2

Pt

Fe2O3

FTO

Figure 6 (a) PEC water splitting system of FeP@Fe2O3 core-shell structure and (b) the energy diagram of the photoanode using FeP assistant for Fe2O3. CB, VB and EF denote the conduction band, valence band, and Fermi level of the Fe2O3, which are collected by the Mott-Schottky and Tauc plots.

29


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Table 1 PEC performances of different photoanodes, including onset potentials (vs. RHE), photocurrents (at 1.23 V vs. RHE), efficiencies of charge separation ηsep and oxidation kinetics ηox (at 1.23 V vs. RHE), and the electron’s transport times τ (at 300 W/m2), respectively. Samples

Onset potentials (V)

Photocurrents (mA/cm2)

ηsep

ηox

τ (ms)

Water/Sulfite oxidation Water/Sulfite oxidation Fe2O3

0.69/0.44

0.21/0.79

11.6%

26.6%

1.25

FeP@Fe2O3-5

0.69/0.43

0.69/1.52

22.4%

45.4%

0.94

FeP@Fe2O3-20

0.69/0.42

0.86/1.70

24.8%

50.6%

0.88

FeP@Fe2O3-80

0.69/0.43

0.55/1.31

19.3%

42.0%

0.99

30


Manipulation of Charge Transfer in FeP@Fe2O3 CoreâShell

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