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Ultrasmall NiFe-phosphate nanoparticles incorporated #-Fe2O3 nanoarrays photoanode realizing high efficient solar water splitting Guang Liu, Yong Zhao, Kaifang Wang, Dongying He, Rui Yao, and Jinping Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03804 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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ACS Sustainable Chemistry & Engineering

Ultrasmall incorporated

NiFe-phosphate α-Fe2O3

nanoparticles

nanoarrays

photoanode

realizing high efficient solar water splitting Guang Liu, Yong Zhao, Kaifang Wang, Dongying He, Rui Yao, Jinping Li∗ Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Research Institute of Special Chemicals, Taiyuan University of Technology, No.79 West Street Yingze, Taiyuan 030024, Shanxi, PR China. KEYWORDS Hematite; photoanode; NiFe-phosphate; synergistic effects; solar water splitting. ABSTRACT The practical application of hematite (α-Fe2O3) in solar water splitting is severely limited by the highly charge recombination rate though its abundant reserves and suitable bandgap of ~2.1 eV. This work describes the synthesis of ultrasmall NiFe-phosphate (NFP) nanoparticles incorporated α-Fe2O3 nanoarrays photoanode via a facile dip-coating and annealing process to demonstrate combined effects on enhanced photoelectrochemical (PEC) water oxidation. The

∗

Corresponding author. Tel.: +86 351 6010908; fax: +86 351 6111178.

E-mail address: [email protected] (J. Li).

ACS Paragon Plus Environment

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NFP uniformly decorating on the surface of hematite nanorods not only could improve water oxidation kinetics and charge separation efficiency, but also could suppress the charge recombination in company with the surface states passivation. Furthermore, the phosphate (P) in the NFP nanoparticles could also play a synergistic effect on promoting the multiproton-coupled electron transfer (PCET) process for the PEC water oxidation. All of these lead to ~140 mV cathodic shift of onset potential, ~2.3 fold enhancement of the photocurrent and excellent longterm stability at 1.23 VRHE in 0.1 M KOH solution for α-Fe2O3/NFP photoanode. Along with these advantages, the NFP nanoparticles may possess new opportunities for modulating PEC water oxidation performances in hematite and other metal oxide photoanodes. INTRODUCTION α-Fe2O3 has been widely employed as promising photoanode materials for solar water oxidation because of many merits, e.g. earth abundant, low cost, chemical stability and favorable bandgap (1.9~2.1 eV) 1-6. However, the effective PEC water oxidation activity of hematite is dramatically suppressed by its inferior water oxidation kinetics, low charge separation & transfer ability derived from the poor conductivity, short carrier life-time and carrier diffusion length surface trapping states

16-17

7-15

and

. In this regard, numerous efforts have been carried out to overcome

these drawbacks, such as integrating oxygen evolving co-catalysts (OECs, like 3d metal oxides/hydroxides) to improve the sluggish water oxidation kinetics

9-10, 12, 18-23

, nano-

engineering of hematite to increase the specific surface area and shorten the holes diffusion length 7, 24-28, elemental doping (e.g. Ti, Ge, Si, Sn, P) to improve the electronic conductivity and facilitate the charge transfer 11, 29-33 as well as passivating the surface states by surface deposition of metal oxides overlayer 9, 16-17, 21.


2

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Among these strategies, coupling OECs with metal oxides semiconductors have been often performed to assist solar water oxidation, which is definite to be one of the rate-limiting steps for PEC water splitting 4. Therefore, developing an efficient OECs is of particular importance and is the essential issue to break the chain of the inferior PEC water oxidation performances. In contrast to high-cost noble metal oxides (IrO2, RuO2)

34-35

, researches on earth-abundant and

cost-efficiency 3d transition metal oxides/hydroxides are of more practical significance to promote the surface water oxidation kinetics of hematite photoanodes metal-based OECs have been widely researched for water splitting

37

36

. Though lots of 3d

, only appropriate cases

have been successfully implanted into PEC water oxidation reactions under simulated sun-light because of a variety of reasons, i.e. different deposition methods, complicated OECs/hematite interfaces and band-edge pinning effects. Consequently, moderate OECs of CoPi 38-39, CoFeOx 9, NiFeOx 12, NiFeOOH 10, 40-41, etc. were integrated with hematite photoanode to demonstrate dual effects on photocurrent enhancement and photovoltage generation, thus facilitating the PEC water oxidation activity. Recently, metal-phosphides have been emerged as efficient electrocatalysts to exhibit surprising electrocatalytic activities for oxygen evolving reactions (OER) evolving reactions (HER)

46-49

and photo-synthesis

50-52

42-45

, hydrogen

. Particularly, it is evidenced that the

electrocatalytic performances of mono-metal phosphide can be dramatically boosted by the second 3d metals incorporation due to the synergistic electron-coupling between the bi-metals 5355

. It is also worthy to note that the metal-phosphides OECs tend to translate into the

corresponding phosphate-rich metal oxide/hydroxide materials during water oxidation process, which are served as the true active electrocatalysts for superior OER performances addition, phosphorus-doped

29

or metal phosphide/phosphate

17, 20, 50, 58

56-57

. In

incorporated α-Fe2O3


3


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photoanodes have been evidenced to demonstrate enhanced PEC water oxidation performances own to improved electron conductivity or enhanced charge separation efficiency. Though these reports have proved that phosphorus/phosphate can be acted as electron donor or co-catalyst to modulate the PEC water oxidation of hematite, to the best of our knowledge, hematite photoanode combined with binary metal phosphate have ever been explored and the roles of multi-component co-catalyst are not yet fully investigated in PEC water oxidation reactions. With these regards in mind, we describe the in situ integrating of ultrasmall NiFe-phosphate (denotes as NFP) electrocatalyst with α-Fe2O3 photoanode by a facile dip-coating and annealing method. By detailed comparing with the PEC water oxidation performances of α-Fe2O3/NFP photoanode with those of pristine α-Fe2O3 and α-Fe2O3 decorated with Ni-phosphate (denotes as NP), Fe-phosphate (denotes as FP), phosphate (denotes as P) as well as NiFeOx (denotes as NF). It is found that the NFP nanoparticles exhibit combined effects on improved PEC water oxidation activity of hematite, which not only could improve water oxidation kinetics and charge separation on the surface, but also could suppress the charge recombination in company with the surface states passivation. Furthermore, the phosphate (P) in the NFP nanoparticles could also play a synergistic effect on promoting the multiproton-coupled electron transfer (PCET) process for the PEC water oxidation. EXPERIMENTAL SECTION Sample synthesis α-Fe2O3 photoanodes were fabricated on a FTO glass substrate by a modified procedure according to the literature 13. Typically, a teflon-lined autoclave (50 mL) was filled with 35 mL aqueous solution containing 3 mmol FeCl3·6H2O (pH was adjusted as 1.5 by HCl solution). Then a piece of cleaned FTO glass (washed with acetone, ethanol and deionized water) was immersed


4

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upside down into the solution and heated at 95 ℃ for 6 h. The obtained yellow color of FeOOH film on FTO glass was washed with deionized water thoroughly to remove any residual and followed air-dried at 80 ℃ for 4h. The FeOOH film was subsequently sintered under N2 at 550 ℃ for 2h and further at 750 ℃ for another 20 min. The α-Fe2O3 nanorod arrays was obtained after sintering process. In order to synthesis of the NFP nanoparticles coated α-Fe2O3 photoanodes, NFP precursor ink was prepared at first. In procedure, 0.9 mmol NaH2PO2 was added into the solvent of 100 mL ethanol containing 0.24 mmol NiCl2·6H2O and 0.06 mmol FeCl3·6H2O with 200 mg PEG, followed by ultrasonicated for 30 min to obtain the NFP precursor ink (the concentration CNi+Fe was 3 mM). Then α-Fe2O3/NFP photoanode was fabricated by a dip-coating and annealing method. First, α-Fe2O3/NFP photoanode was soaked in 3 mM NFP precursor ink for 1 min and then the wet film was air-dried at 60 ℃ for 2 h, then the film was further annealed under N2 at 300 ℃ for another 1 h. Finally, the as-prepared photoanode was thoroughly rinsed by deionized water and ethanol to remove any residual. In contrast, NP, FP, P and NF coated hematite photoanodes were fabricated by the same procedure. In addition, different concentrations of NFP precursor ink (1, 3, 5, 10 mM) were also examined to optimize the best NFP loading amount. Characterization The structure and morphology were investigated by X-ray diffractometer (XRD, Cu Kα radiation, Bruker D8 Advance), Raman spectrometer (514.5 nm excitation argon laser, Renishaw inVia), scanning electron microscopy (SEM, SU8010 Hitachi), energy-dispersive X-ray spectroscopy (EDS, SDD-2610 IXRF) and transmission electron microscopy (TEM, JEOL2010FEF). The chemical composition for these photoanodes were studied by X-ray


5


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photoelectron spectroscopy (XPS, Thermo VG ESCALAB250). Optical diffuse reflection characteristic of these photoanodes was recorded by UV-Visible spectrometer (UV–Vis, L650 PerkinElmer). Photoelectrochemical characterization The PEC water oxidation performances of these photoanodes were evaluated by using a threeelectrode system on electrochemical workstation (VersaSTAT 3, Princeton Applied Research). The as-prepared photoanode was set as the working electrode (sealed by Kapton tape, exposed area ~1 cm2), Pt foil (1×1 cm) and Ag/AgCl (3.5 mol/L KCl solution) were served as the counter electrode and reference electrode, respectively. 0.1 M KOH aqueous solution was employed as electrolyte. An AM 1.5G simulated sun-light irradiation lamp (Perfectlight, LS-SXE300CUV, power intensity was calibrated to 100 mW/cm2 by a radiometer of SEAWARD Solar Survey 100) was selected as light source and all the samples were illuminated from the back-side. I–V curves of the samples were performed at scan rates of 20 mV/s and all the potentials were normalized

to

the

potential

of

reversible

hydrogen

electrode

(RHE,

ERHE=EAg/AgCl+0.059pH+0.205). Electrochemical impedance spectroscopy (EIS) was recorded at 1.23 VRHE with amplitude of 5 mV in the range of 105 Hz~10-1 Hz under AM 1.5G light irradiation (100 mW/cm2). The applied bias photo-to-current efficiency (ABPE) was calculated by the following equation (1) 18: ABPE =

× . | |

× 100%

(1)

Where J is the photocurrent density at applied bias VRHE, Plight is the power intensity of 100 mW/cm2. The incident photon-to-electron conversion efficiency (IPCE, 340~700 nm, 1.23 VRHE)


6

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was recorded by an IPCE system (Zolix Solar Cell Scan 100) and calculated as the following equation (2) 29: IPCE =

!"#" $

× 100%

(2)

Where J is the photocurrent density measured at wavelength of λ, Pmono is the monochromatic illumination power (mW/cm2). Mott–Schottky tests were determined under a frequency of 103 Hz at a scan rate of 10 mV/s in 0.1 M KOH solution (pH=12.6). The donor concentration (Nd) and flat band potential (Vfb) can be calculated as following formula 50, 59:

%&

= 'εε

( )*

V − V-. −

N2 = 3 5 6d 3 & 5 /dV9 '44 %

/0 '

(

(3)

(4)

Where C denotes the space charge region's capacitance, e is electron charge and equal to 1.6×1019

C, ɛ≈80 represents the dielectric constant of hematite, the vacuum permittivity of ɛ0 is

8.85×10-14 F/cm and V is the applied potential on the photoanode. I-V plots in 0.1 M KOH solution containing 0.5 M Na2SO3 as a hole scavenger were recorded to determine the charge separation efficiency in the bulk (ηbulk) and on the surface (ηsurface). Typically, the photocurrent density of hematite photoanode derived from PEC performance (JPEC) can be defined as following 9: JPEC=Jabs×ηbulk×ηsurface

(5)


7


Page 8 of 31

It suggests that Na2SO3 oxidation reaction is more thermodynamically and kinetically facile than that of water oxidation, and adding Na2SO3 as a hole scavenger can maximum suppress the charge recombination on the surface (ηsurface assumed to be 100%) without impressing the charge separation efficiency in the bulk photoanode. Therefore, ηbulk and ηsurface can be defined as following 29: η.;& ?@A /J>.B

(6)

ηB;C->D' = JE& @ /J)>&?@A

(7)

Where J)>& ?@A and JE& @ are the photocurrent density for Na2SO3 oxidation and PEC water oxidation, Jabs is the photocurrent density assuming the absorbed photons completely convert into current and calculated to be 10.6 mA/cm2 in this work by integrating the overlapped area between UV-Vis spectrum and standard ASTM G-173–03 spectrum. Besides that, a gas chromatography (Shimadzu, GC-2014C) was employed to measure the total evolved O2 amounts under long-term PEC reaction at 1.23 V vs. RHE. RESULTS AND DISCUSSION


8

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Scheme 1. Schematic illustration of the fabrication process of α-Fe2O3/NFP photoanode. (a) αFe2O3 nanorods grown on FTO substrate by hydrothermal method (95 ℃, 6 h) and followed annealing treatment (N2, 550 ℃ for 2 h and further 750 ℃ for 20 min); (b) NFP precursor was coated on the α-Fe2O3 nanorod arrays by dip-coating process; (c) Ultrasmall NFP nanoparticles were decorated on the α-Fe2O3 nanorod arrays after another annealing process (N2, 300 ℃ for 1 h). Scheme 1 shows the schematic illustration of fabrication process for α-Fe2O3/NFP photoanode. First, α-Fe2O3 nanorod arrays were grown on FTO substrate through hydrothermal method and followed by annealing process (Scheme 1a). It is well accepted that one-dimensional hematite nanoarrays are effective in charge transfer by shortening the carriers diffusion pathway. Then the


9


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Figure 1. (a) Top-view and (b) side-view SEM images of α-Fe2O3 nanoarrays. (c) Top-view and (d) side-view SEM images of α-Fe2O3/NFP nanoarrays. (e) XRD patterns and (f) Raman spectra of α-Fe2O3 and α-Fe2O3/NFP nanoarrays. as-prepared α-Fe2O3 nanorod arrays were coated with NFP precursor via solution infiltration (Scheme 1b). Finally, ultrasmall NFP nanoparticles were decorated on the surface of the α-Fe2O3 nanorod arrays along with a facile thermal treatment (Scheme 1c, under N2 at 300 ℃ for 1 h). The coupling of OECs on hematite photoanodes to improve the surface OER kinetics is a


10

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widely-accepted strategy in PEC water oxidation reactions. Though the evident benefited PEC water oxidation kinetics of hematite photoanode have been realized by CoPi

38

and NiPi

20

, the

binary metallic phosphide/phosphate decorated on hematite photoanode have ever been touched, especially the binary metallic phosphide/phosphate were demonstrated much better OER activities than that of individual one 57. Therefore, we aim to precisely and uniformly deposit the ultrasmall NFP nanoparticles on hematite nanorod nanoarrays to realize combined effects on enhancing PEC water oxidation kinetics, facilitated charge transportation and passivated surface states, thus leading to promote the PEC performances. The morphology of α-Fe2O3 photoanode presents a one-dimension nanorod structure in the SEM image (Figure 1a) and the thickness of α-Fe2O3 film is determined to be 300±50 nm (Figure 1b). The α-Fe2O3/NFP photoanode demonstrates a similar nanorod-like morphology with a more rougher surface (Figure 1c), the ultrasmall NFP nanoparticles decorated on the nanorods can be observed evidently from both the top-view and side-view of SEM image (Figure 1d). The XRD patterns of α-Fe2O3 and α-Fe2O3 decorated with NFP, NP, FP, P samples demonstrate similar Bragg diffraction peaks ascribed to SnO2 (PDF no. 46-1088) from FTO substrate and hematite (PDF no. 33-0664), the absence of XRD pattern of NFP could be due to the amorphous feature of decorated ultrasmall nanoparticles (Figure 1e and Figure S1a). In addition, all the six peaks in the Raman spectra of α-Fe2O3 and α-Fe2O3 decorated with NFP, NP, FP, P samples (Figure 1f and Figure S1b) can be ascribed to hematite 7, also indicating the successfully synthesis of hematite photoanodes. The existence of ultrasmall NFP nanoparticles can be further verified by the comparison of TEM and HR-TEM images of α-Fe2O3 (Figure S2a and b) and α-Fe2O3/NFP sample (Figure 2a and b). Both the α-Fe2O3 (Figure S2b) and αFe2O3/NFP (Figure 2b) samples in HR-TEM show high crystalline structure with lattice spacing


11


Page 12 of 31

of 0.27 nm and 0.37 nm according to the (104) and (012) planes of hematite. The ultrasmall NFP nanoparticles with a diameter of ~10 nm can be observed distinctly on the surface of αFe2O3/NFP sample (Figure 2b). Moreover, the absence of any lattice spacing for crystal structure further confirmed the amorphous feature of NFP nanoparticles, which is also consistent with the result of XRD patterns. Besides, according to the EDS data (Figure S3), the molar ratio of Ni:P is calculated to be 0.83:1, in consideration of the molar ratio of NiCl2•6H2O: FeCl3•6H2O in the original materials is 4:1, so the (Ni+Fe):P ratio is close to 1:1 in NiFe-PO4 (denotes as NFP). The various metal-phosphate nanoparticles loading on α-Fe2O3 nanorod was also investigated by the electronic states and surface composition of XPS spectrum (Figure 2c-f, Figure S4). The high-resolution XPS peaks (Figure 2c) of Fe 2p1/2 and Fe 2p3/2 located at 724.1 eV and 710.8 eV as well as satellite peak at 718.5 eV confirm the existence of Fe3+ in these two photoanodes 60

. In addition, the positive shift of 0.5 eV in the Fe 2p peak of α-Fe2O3/NFP photoanode can be

determined compared with that of pristine α-Fe2O3 photoanode due to the electronic interaction between NFP nanoparticles and hematite. The binding interaction between NFP nanoparticles and hematite can be further evidenced by the presence of P-O (531.1 eV) bond of phosphate in the fitted O 1s XPS peak (Figure 2e) 17. The deconvolution of P 2p spectrum (Figure 2f) for αFe2O3/NFP photoanode presents two peaks at 134.1 eV (P 2p1/2), 133.2 eV (P 2p3/2), which can be ascribed to P5+ in PO43- 53. According to the results of XPS spectrum, it can be concluded that the NFP nanoparticles are composed of NiFe-PO4, and the molar ratio of Ni:P is determined to


12

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Figure 2. (a) TEM and (b) HRTEM images of α-Fe2O3/NFP nanoarrays. High-resolution XPS spectrum of (c) Fe 2p, (d) Ni 2p, (e) O 1s and (f) P 2p for α-Fe2O3/NFP nanoarrays. be 0.85:1, which is also consistent with that of the result of EDS data. Furthermore, the UV-Vis light-absorbance spectrum (Figure 3) demonstrates the similar absorption edges at ~610 nm for α-Fe2O3 and α-Fe2O3/NFP photoanodes, which implies that the decorated NFP nanoparticles have a negligible effect on the light-absorption of the hematite photoanode 50.


13


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Figure 3. UV–Vis diffuse reflection spectra of α-Fe2O3 and α-Fe2O3/NFP nanoarrays. In order to optimize the best NFP loading amount for promoted effects on PEC water oxidation performance, photocurrent density versus potential (I–V) curves of the photoanodes decorated with different NFP precursor concentrations were recorded in 0.1 M KOH solution under AM 1.5G illumination. As shown in Figure S5, the increase of NFP precursor concentrations from 1 mM to 3 mM corresponds to both an increased photocurrent density and a reduced turn-on potential. However, the continue increase of NFP precursor concentrations greater than 3 mM leads to a negative effect on both photocurrent density and turn-on potential, which indicating that the concentration of 3 mM for NFP precursor is the optimized NFP nanoparticles loading amount. With the best NFP loading amount, additional I-V curves (Figure 4a) and turn-on potentials (Von, Figure 4b) were recorded for pristine α-Fe2O3 and α-Fe2O3 decorated with various co-catalysts (NFP, NP, FP, P) photoanodes. As shown in Figure 4a, the photocurrent density demonstrates a dramatically increase from 0.53 mA/cm2 to 1.2 mA/cm2 at 1.23 VRHE with the trend of pristine α-Fe2O3 < α-Fe2O3/P < α-Fe2O3/FP < α-Fe2O3/NP < αFe2O3/NFP, indicating the improved PEC water oxidation kinetics upon co-catalysts coating.


14

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Figure 4. (a) I-V curves, (b) butler plots, (c) ABPE plots and (d) IPCE plots of bare Fe2O3 and αFe2O3 NRs photoanodes loaded with different catalysts in 0.1 M KOH solution under AM 1.5G illumination (100 mW/cm2). Additionally, the Von derived from Butler plots, shown as Figure 4b, where the Von cathodic shift by 20 mV (from 0.88 to 0.86 VRHE) for α-Fe2O3/FP, 30 mV (from 0.88 to 0.85 VRHE) for αFe2O3/NP and further 140 mV (from 0.88 to 0.74 VRHE) for α-Fe2O3/NFP photoanode, respectively. Whereas the Von of α-Fe2O3/P photoanode exhibits anodic shift by 60 mV from 0.88 to 0.94 VRHE, demonstrating the different roles of NP, FP and P coating. It has been reported that phosphate on the surface can act as a labile ligand that facilitate the proton release from water, thus benefit for the PCET step 45, 57, ultimately enhancing the OER kinetics, which is also verified by the improved PEC performance of α-Fe2O3/NFP photoanode compared with that of α-Fe2O3/NF photoanode in the dark and illumination (Figure S6). The applied bias photon–to– current efficiency (ABPE, assuming a 100% Faradaic efficiency, Figure 4c) also demonstrates that the α-Fe2O3/NFP photoanode achieves the maximum ABPE value of 0.17 % at 0.98 VRHE,


15


Page 16 of 31

which is 5.6 times higher than that of the pristine α-Fe2O3 (0.03% at 1.08 VRHE). The enhanced photon-to-electron conversion efficiency (IPCE, 28.7% at 340 nm) of α-Fe2O3/NFP photoanode is also consistent with the highest photocurrent density. This performance of α-Fe2O3/NFP photoanode can be comparable of those of the best hematite photoanodes coupled with OECs 18, 21, 50

8,

. Therefore, in consideration of the highest photocurrent at 1.23 VRHE and lowest Von

potential upon NFP loading as well improved ABPE and IPCE values, it can be hypothesized that the ultrasmall NFP nanoparticles present a synergistic effect on enhancing the PEC performance of hematite. The promoted photocurrent density can be ascribed to the increased electrochemical surface area of the solid-liquid junction by forming hematite/NFP interface, which can be evidenced by determining the double layer capacitance (Cdl, scan rate dependent CV curves were conducted between 0.9~1.0 VRHE in 0.1 M KOH solution in the dark) of the photoanodes (Figure S7). Obviously, the value of Cdl for the α-Fe2O3/NFP photoanode (6.1 µF/cm2) is nearly twofold larger than that of pristine α-Fe2O3 photoanode (3.3 µF/cm2) because of the dramatically enhanced surface area of the solid-liquid junction by forming hematite/NFP interface. Moreover, the enhancing mechanism of ultrasmall NFP nanoparticles on hematite photoanode was further investigated by electrochemical impedance spectroscopy (EIS) at 1.23 VRHE under illumination (AM 1.5 G). As shown in Figure 5a, two clear semicircles are visible in each Nyquist plot, which can be fitted by a typical equivalent circuit model composes a couple of capacitance and resistances in parallel (Figure 5b). In this case, Rs defines as the series resistance in the PEC cell, Rct,bulk in low impedance (high frequency) represents the resistance in the hematite photoanode, Rct,trap in the high impedance semicircles (low frequency) is according to the charge resistance across the interface of electrolyte/hematite 18. It is found that all the series resistance of


16

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co-catalysts loading photoanodes are almost the same as the pristine α-Fe2O3 photoanode (61.8~73.4 Ω, Table 1), suggest the interface between hematite and FTO substrate unchanged. A dramatically decrease in the charge transfer resistance of Rct,trap from 3368.1 to 270.6 Ω upon the loading of NFP nanoparticles indicates that the photogenerated holes on α-Fe2O3/NFP photoanode can inject into electrolyte more easily than that of pristine α-Fe2O3 photoanode. Furthermore, it is indicated that α-Fe2O3/NFP (467 Ω), α-Fe2O3/NP (473.9 Ω), α-Fe2O3/FP (479.6 Ω) and α-Fe2O3/P (483.8 Ω) photoanodes demonstrate comparable enhanced conductivity (Rct,bulk) in contrast with pristine α-Fe2O3 (528.3 Ω) photoanodes (Table 1) ascribing to the metal phosphate incorporation, further suggest that phosphate has different role on improving the PEC water oxidation. Accordingly, it is well accepted that the conductivity of the bulk hematite photoanode could not change by the surface decoration of co-catalysts

21

, the improved charge

transfer in the bulk might be came from the facilitated charge separation, proposing that NFP nanoparticles could act additional effects on improving PEC water oxidation besides surface catalytic effect. Table 1. EIS results of these hematite photoanodes in this work. Samples

Rs

Rct,bulk

Rct,trap

α-Fe2O3/NFP

61.8

467

270.6

α-Fe2O3/NP

64.9

473.9

418.1

α-Fe2O3/FP

72.1

479.6

565.2

α-Fe2O3/P

65.7

483.8

1744.2

α-Fe2O3

73.4

528.3

3368.1


17


Page 18 of 31

Recently, it is found that surface trapping states of hematite photoanode can be suppressed by incorporation of metal oxides co-catalysts (for example, NiFeOx, CoFex)

9, 12

in addition to

traditional passivation layers of Al2O3, so as to result in enhancing the photovoltage and lowering the photocurrent response. To verify this deduction, the difference in open-circuitpotentials (Von=Vdark-Vlight, Figure S8a and b) were recorded to determine the steady-state opencircuit photovoltage (OPV, Figure S8c). Because of the more positive Von in the dark (Figure S8c), a larger photovoltage of 0.17 V for α-Fe2O3/NFP photoanode is achieved upon the coating

Figure 5. (a) EIS plots of bare Fe2O3 and α-Fe2O3 NRs photoanodes loaded with different catalysts; (b) equivalent circuit of these photoanodes.


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of NFP nanoparticles compared with that of bare α-Fe2O3. Typically, the significantly positive Vdark with NFP loading indicates a more pronounced upward band bending up to passivated surface states, therefore, the improved photovoltage of α-Fe2O3/NFP in contrast with bare αFe2O3 photoanode can be ascribed to the surface states passivation. This is can be also evidenced by comparison of the CV curves for α-Fe2O3/NFP and pristine α-Fe2O3 photoanodes in 0.1 M KOH solution in the dark. As shown in Figure S9, the cathodic peak corresponding to the reduction of surface Fe (III) at 1.1~1.3 VRHE was disappeared after decorating NFP nanoparticles, which also indicates the surface states are effectively suppressed. Additionally, the enhanced photovoltages by the passivation effect of NFP decoration can be further evidenced by the 80 mV cathodic shift of flat-band potential (EFB) in the Mott–Schottky plots of α-Fe2O3/NFP and bare α-Fe2O3 photoanode (Figure 7a). Moreover, because of the more thermodynamically and kinetically facile photo-oxidation of Na2SO3 than water, the cathodic shift of photocurrent onset in I-V curves obtained from PEC Na2SO3 oxidation of α-Fe2O3/NFP and bare α-Fe2O3 photoanode (Figure S10) can further prove the improved photovoltage originated from passivation of surface states.

Figure 6. Charge separation efficiency (a) in the bulk (ηbulk) and (b) on the surface (ηsurface) of bare Fe2O3 and α-Fe2O3/NFP NRs photoanode in 0.1 M KOH solution.


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Furthermore, I–V measurement of PEC Na2SO3 oxidation in Figure S10 can be further used to verify the additional charge separation effects derived from NFP incorporation. As mentioned above, the reaction kinetics of PEC Na2SO3 oxidation is more fast for α-Fe2O3/NFP than that of the pristine α-Fe2O3 photoanode. In this case, the charge separation efficiency in the bulk (ηbulk, Figure 6a) derived from equation (6) for α-Fe2O3/NFP (~19.5% at 1.6 VRHE) is gradually higher than that of bare α-Fe2O3 photoanode (~12.3% at 1.6 VRHE), indicating the enhanced bulk activity can be ascribed to the surface states passivation and increased electrochemical active area on the surface. Additionally, the charge separation on the surface efficiency (ηsurface) is also calculated according to equation (7), as shown in Figure 6b. The ηsurface for α-Fe2O3/NFP is also higher than that of bare α-Fe2O3 photoanode in the whole potential range (0.6-1.6 V) and attains ~78% at 1.6 VRHE, implying that severe surface charge recombination is reduced, which may

Figure 7. (a) Mott–Schottky plots and (b) stability tests of bare Fe2O3 and α-Fe2O3/NFP NRs photoanode in 0.1 M KOH solution (measured at 1.23 V vs. RHE, AM 1.5G illumination).


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originate from the synergistic effects of improved surface water oxidation kinetics, facilitated charge separation efficiency and passivated surface states upon NFP loading. The charge separation effect caused by decoration of NFP nanoparticles was also conducted by Mott–Schottky analysis (Figure 7a). The Mott–Schottky plots for both α-Fe2O3/NFP and bare α-Fe2O3 photoanode exhibit positive slopes, suggests the n-type semiconductor feature of these two hematite photoanodes 29. While the flat-band potential (EFB, cathodic shift of 80 mV) and the slope of Mott–Schottky plot are evident decrease after NFP loading, implies the increment of carrier density (Nd, derived from the slope of Mott–Schottky plots using equation (3) and (4)) 29. The value of Nd increases from 5.6×1019 cm-3 to 3.5×1020 cm-3 by the incorporation of NFP nanoparticles. It is well known that the increment of carrier density can increase the band bending at the interface of hematite/electrolyte (Scheme 1) 61, thus facilitating the charge transfer 21

. Hence, the approximate one-order higher carrier density of α-Fe2O3/NFP photoanode

indicates that the carrier conductivity of NFP nanoparticles is much higher than that of hematite substrate, which is also consistent with the result of PEC Na2SO3 oxidation test, further suggesting the charge separation and holes inject into electrolyte more effectively. Above all, the highly efficient PEC water oxidation performance of α-Fe2O3/NFP photoanode is attributed to the synergistic effects of NFP decoration, which not only could improve water oxidation kinetics and charge separation efficiency, but also could suppress the charge recombination in company with the surface states passivation. To further evaluate the enhanced PEC water oxidation performance, 5.5 h stability tests were also carried out at 1.23 VRHE under AM 1.5G illumination in 0.1 KOH solution. As shown in Figure 7b, both α-Fe2O3/NFP and bare α-Fe2O3 photoanode present acceptable stability within testing duration. In the given 5.5 h experimental time duration, α-Fe2O3/NFP photoanode


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demonstrates an impressive performance of photostability and attains a stable photocurrent density of ~1.2 mA/cm2 without declining. Whereas the photocurrent density exhibits a distinct decrease from 0.54 mA/cm2 to 0.47 mA/cm2 for pristine α-Fe2O3 photoanode. Furthermore, the Faradic efficiency of the evolved oxygen for α-Fe2O3/NFP photoanode was determined to be 92~96% within the first 90 minutes (Figure S11). SEM (Figure S12a) and TEM (Figure S13) images of α-Fe2O3/NFP photoanode after stability testing confirm the surface NFP nanoparticles complete evolve into 2~3 nm thick nanolayers surrounding on the surface of hematite nanorods, which is ascribed to the partially dissolution or exfoliation of the phosphate in electrolyte during stability testing, as evidenced by the reduction of P signal after long-term stability testing (Figure S12b and Figure S14). These results highlight the critical role of the NFP nanoparticles in avoiding photo-corrosion and promoting the PEC photostability of α-Fe2O3 PEC photoanodes. CONCLUSIONS In brief, we have described the enhanced PEC water oxidation performance of hematite by coupling with ultrasmall NFP nanoparticles. Owning to the synergistic effects of improve water oxidation kinetics, efficient charge separation as well as surface states passivation by decoration of NFP nanoparticles during PEC water oxidation process, the turn-on potential cathodic shift by 140 mV to 0.74 VRHE, and the photocurrent density increases from 0.53 mA/cm2 to 1.2 mA/cm2 at 1.23 VRHE in 0.1 M KOH solution. The α-Fe2O3/NFP photoanode demonstrates an impressive performance of photostability and attains a stable photocurrent density of ~1.2 mA/cm2 for more than 5 h at 1.23 VRHE without declining. Additionally, the phosphate in the NFP nanoparticles could also play a vital role in promoting the multiproton-coupled electron transfer (PCET) process for the PEC water oxidation. With these merits, the NFP nanoparticles could provide


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new opportunities for modulating PEC water oxidation performances in hematite and other metal oxide photoanodes. ASSOCIATED CONTENT Supporting Information. XRD patterns, Raman spectra, TEM images, XPS spectrum, control PEC characterizations, SEM, TEM and XPS analyses of α-Fe2O3/NFP photoanode after stability test. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Funding Sources We thank the financial funding supported by Natural Science Foundation of China (51402205), Natural Science Foundation of Shanxi (2015021058) and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (STIP-2016131). REFERENCES (1)

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Owning to the synergistic effects of improve water oxidation kinetics, efficient holes injection and surface states passivation, α-Fe2O3/NFP photoanode demonstrates much enhanced PEC water oxidation performances by ultrasmall NFP loading.


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Ultrasmall NiFe-Phosphate Nanoparticles ... - ACS Publications

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