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Quasi-Topotactic Transformation of FeOOH Nanorods to Robustness Fe2O3 Porous Nanopillars Triggered with a Facile Rapid-Dehydration Strategy for Efficient Photoelectrochemical Water Splitting Aizhen Liao, Huichao He, Lanqin Tang, Yichang Li, Jiyuan Zhang, Jiani Chen, Lan Chen, Chunfeng Zhang, Yong Zhou, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00367 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Materials & Interfaces

Quasi-Topotactic Transformation of FeOOH Nanorods to Robustness Fe2O3 Porous Nanopillars Triggered with a Facile Rapid-Dehydration Strategy for Efficient Photoelectrochemical Water Splitting Aizhen Liao,a,d Huichao He,b Lanqin Tang,a,e Yichang Li,a Jiyuan Zhang,f Jiani Chen,c Lan Chen,a Chunfeng Zhang,a Yong Zhou,a,f and Zhigang Zoua,d,e, f a

National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Physics, Nanjing University, Nanjing 210093, P. R. China. b

State Key Laboratory of Environmental Friendly Energy Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan 621010, P. R. China. c

State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, P. R. China. d

School of Engineering and Applied Science, Nanjing University, Nanjing 210093, P. R. China.

e

College of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 22401, P. R. China. f

Kunshan Innovation Institute of Nanjing University, Kunshan, Jiangsu 215347, P. R. China.

Abstract A facile rapid dehydration strategy (RD) is explored for quasi-topotactic transformation of FeOOH nanorods to robust Fe2O3 porous nanopillars, avoiding collapse, shrink, and coalescence, compared with a conventional treatment route. Additionally, the so-called RD process is capable of generating beneficial porous structure for PEC water oxidation. The obtained RD-Fe2O3 photoanode exhibits a photocurrent density as high as 2.0 mA cm-2 at 1.23 V vs. RHE, and a saturated photocurrent density of 3.5 mA cm-2 at 1.71 V vs. RHE without any co-catalysts, which is about 270% improved photocurrent density over Fe2O3 with conventional temperature-rising route (0.75 mA cm-2 at 1.23 V vs. RHE and 1.48 mA cm-2 at 1.71 V vs. RHE, respectively). The enhanced photocurrent on RD-Fe2O3 is attributed to a synergistic effect of following factors: (i) preservation of single crystalline nanopillar decreases charge carriers recombination; (ii) the formation of long nanopillars enhance light harvesting; (iii) porous structure shortens hole transport distance from 1

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bulk material to electrode/electrolyte interface. Keywords: Rapid dehydration; Quasi-topotactic transformation; Robustness Fe2O3 nanopillars; Porous Structure; Photoelectrochemical Water Splitting

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1. Introduction Converting solar energy into hydrogen via photoelectrochemical (PEC) water splitting is promising for sustainable energy supply.1-3 The PEC water splitting cell is composed of two half reactions: the oxygen evolution reaction (OER) on the anode and the hydrogen evolution reaction (HER) on the cathode.4 The OER involves four holes for each O2 molecule on photoanode, which is the rate-determining step for PEC water splitting.5, 6 Adequate light absorption, effective charge separation and transfer are expected to achieve high photocurrent on photoanodes.7, 8 In this regard, hematite (α-Fe2O3), the most stable Fe2O3 phase under ambient condition with wide application such as gas-sensor, lithium-ion batteries, is explored as one promising candidate photoanode due to its intrinsic advantages of non-toxic, abundant, small bandgap (~2.1 eV), favorable band edge positions, and good aqueous stability.9-11 α-Fe2O3 photoanode could achieve a maximum theoretical solar-to-hydrogen (STH) efficiency of ~15.8%.12,

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However, its short hole-diffusion length, high electron-hole

recombination rate, and slow charge-transfer kinetics, greatly restrict its PEC application.14-18 Morphological engineering of one-dimensional (1D) nanostructures including nanowires, nanotubes, and nanorods with high aspect ratios and large surface areas can improve the charge carrier collection of photoanode through minimizing hopping transport, and thus reducing recombination losses at grain boundaries. Moreover, 1D Fe2O3 with smaller diameters can also minimize the distance needed for hole diffuse to electrolyte-semiconductor interface, thereby avoiding the poor charge transport limitation.19 FeOOH nanorods grown on conductive substrates are generally utilized as precursors for the fabrication of Fe2O3 nanorods through a dehydration process with high-temperature thermal treatment. Unfortunately, such thermal annealing process always leads to collapse, shrink, and coalescence of resulting Fe2O3 1D structure, which significantly impacts the light absorption capability and the charge transport pathway of Fe2O3 photoanode.20-23 Most recently, a silica encapsulation method was introduced to retain Fe2O3 nanowire morphology even after high temperature calcination at 800 oC.24, 25 In addition, the 3



FeOOH nanorod precursor covered by a ZrO2 shell can also withstand high-temperature structural collapse, and produce robust Fe2O3 nanotubes through analogous Kirkendall effect.26 In this work, a facile rapid dehydration (RD) strategy is explored for quasi-topotactic transformation of FeOOH nanorods to robust Fe2O3 porous nanopillars. The FeOOH nanorods array is directly placed into high-temperature furnace to induce fast dehydration, instead of conventional slow temperature-rising process. The so-called RD process is capable of not only retaining the morphology of FeOOH nanorods precursor, alleviating structural collapse and agglomeration or coalescence of Fe2O3 nanopillars, but also generating beneficial porous structure for PEC water oxidation (the corresponding sample is abbreviated as RD-Fe2O3). The obtained RD-Fe2O3 photoanode exhibits a photocurrent density as high as 2.0 mA cm-2 at 1.23 V vs. RHE, and a saturated photocurrent density of 3.5 mA cm-2 at 1.71 V vs. RHE without any co-catalysts, which is about 270% improved photocurrent density over Fe2O3 with conventional temperature-rising route (the corresponding sample is abbreviated as C-Fe2O3) (0.75 mA cm-2 at 1.23 V vs. RHE and 1.48 mA cm-2 at 1.71 V vs. RHE, respectively). Additionally, the RD-Fe2O3 photoanode shows a cathodically shifted onset potential by about 80 mV from 1.03 to 0.95 V vs. RHE, and good PEC reaction stability, maintaining for 5 h in alkaline electrolytes without deterioration. The enhanced photocurrent on RD-Fe2O3 is attributed to a synergistic effect of following factors: (i) preservation of single crystalline nanopillar decreases charge carriers recombination; (ii) the formation of long nanopillars enhance light harvesting; (iii) porous structure shortens hole transport distance from bulk material to electrode/electrolyte interface.

2. Experimental section Sample Preparation C-Fe2O3 nanorod was synthesized according to the recipe reported by our group.27 In brief, 20 mL aqueous solution containing 0.15 M ferric chloride (FeCl3.6H2O, 99.99%), 0.3 M urea (CO(NH2)2, AR, 99%) and 20 µL 4


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titanium trichloride (TiCl3, AR, 15.0-20.0% TiCl3 basis in 30% HCl) was added into a Teflon-lined stainless steel autoclave and heated at 100 oC for 6 h. The obtained FeOOH nanopillar was slowly heated-up from room temperature to 550 oC for annealing 120 min in a muffle furnace in air, and subsequently at 650 oC for 30 min to completely transform FeOOH into Fe2O3 nanorod. Different from the conventional method, for the fabrication of RD-Fe2O3, the FeOOH was directly put into a 250 oC furnace for annealing 8 min instead of the heating up from room temperature, followed by the same annealing process with the conventional route. Characterizations An FEI NOVA NanoSEM230 scanning electron microscope was employed to investigate the morphology of samples. The crystal structure of samples was identified by X-ray diffraction (XRD) (Ultima III, Rigaku) with Cu Ka radiation (k = 0.154 nm). Transmission electron microscope (TEM) images were taken on a JEM 200CX TEM apparatus. X-ray photoelectron spectroscopy (XPS) was carried out

on

a

Thermo

Scientific

K-Alpha

instrument

operating

with

an

unmonochromatized Al Ka X-ray source, and the data were calibrated by the binding energy of the C1s line at 283.6 eV. A Shimadzu UV-2550 spectrometer equipped with an integrating sphere was used to investigate the absorption properties of samples. Transient PL decay spectra were collected using the time-correlated singlephoton counting technique (Picoharp 300). For femtosecond transient absorption (TA) spectroscopy, pulses at 350 nm was used as the pump source. Electrochemical impedance spectroscopic (EIS) curves were measured by a PAR2273 workstation (Princeton Applied Research, USA) under a forward bias of 0.2 V and AM 1.5G illumination. The frequency ranged from 0.1 mHz to 100 kHz. Photoelectrochemical property measurements The photoelectrochemical (PEC) performance of the photoanodes was investigated in a three-electrode cell using an electrochemical analyzer (CHI-630D, Shanghai Chenhua) under AM 1.5G illumination (standard 100 mW cm-2) cast by an Oriel 92251A-1000 sunlight simulator calibrated by the standard reference of a Newport silicon solar cell. The electrolyte was a 1 M NaOH aqueous solution (pH 13.6). The Fe2O3 sample was used 5



as a working electrode. A Pt foil and a saturated Ag/AgCl electrode were used as a counter and a reference electrode. The RHE potential was calculated following the formula VRHE = VAg/AgCl + 0.059pH + EAg/AgCl, where VRHE is the converted potential versus RHE, and EoAg/AgCl = 0.1976 V at 25 oC. The active area of the Fe2O3 sample was fixed to 0.28 cm2 using a black mask. A cyclic voltammetry method was adopted with a scan rate of 10 mV s-1.

3. Results and Discussion Figure 1 shows a schematic illustration of the preparation of C-Fe2O3 and RD-Fe2O3. FeOOH is firstly hydrothermally grown on FTO glass, showing a dense, prism-shaped ordered array with an average diameter and length of 50 nm and 400 nm, respectively [Figure 2(a-c) and Figure S1(a, d)]. For the fabrication of C-Fe2O3, the FeOOH nanorod is slowly heated-up from room temperature to 550 oC in air at a rate of 5 oC/min, maintaining at 550 oC for 2 h, and subsequently at 650 oC to completely transform FeOOH into Fe2O3 nanorod. The produced Fe2O3 displays disorderly dense and coalesced morphology. The diameter and length were shrunk to 20-40 nm and 280 nm, respectively [Figure 2(d-f) and Figure 3(a, b)]. For the fabrication of RD-Fe2O3, the FeOOH nanorod is directly put into a 250 oC furnace instead of the slowly heating up from room temperature, followed by the same annealing process with the conventional route. The produced RD-Fe2O3 well preserves the prism morphology and dimension of an average diameter of 50 nm and 390 nm in length [Figure 2(g-i)], exhibiting quasi-topotactic transformation with minor structural modification. Notably, some nanopores are generated in the RD-Fe2O3 during the thermal transformation [Figure 3(c, d)], which is normally expected to be beneficial to shorten holes transport distance from bulk material to electrode/electrolyte interface. The RD-Fe2O3 with a lattice spacing of 0.27 nm matches a (100) plane, indicating a longitudinal growth direction of the single crystal nanopillar along [100] [Figure 3(e, f)]. The TEM mapping analysis of Fe and O confirms the uniform dispersion of each element in RD-Fe2O3 [Figures 3(g-i)]. The XRD patterns show characteristic 6


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diffraction peaks of hematite (Fe2O3; JCPDS 79-0007) (Figure S2). The Fe2p XPS spectrum exhibits two major peaks at 710.5 eV for Fe2p3/2 and 724.3 eV for Fe2p1/2. The binding energies of both peaks match well with the characteristics of Fe3+ in Fe2O3. A small peak at 718.5 eV can also be precisely assigned to Fe3+ rather than enriched oxygen vacancies. The reason is that the annealing process was operated at air atmosphere. The formation possibility of enriched oxygen vacancies is less. The O1s peak can be fitted into two main constituent peaks centered at 529.7 and 531.8 eV (Figure S3b), which correspond to O2- and OH-, respectively. Thermogravimetric (TG) analysis of the FeOOH precursor shows gradual weight-loss before 150 °C, corresponding to the elimination of adsorbed water (Figure S1h). A sharp weight loss is observed in the range of 160 oC to 300 oC, which is assigned to dehydration of -OH groups.28 The corresponding XRD pattern also confirms that part FeOOH starts transformation into of Fe2O3 with direct annealing at 250 oC (Figure S1g). The reason that the conventional transformation route of FeOOH to Fe2O3 always generates distorted dense Fe2O3 nanorods may be ascribed to relatively low melting point of FeOOH of 350-400 oC.29 Slow heating may gradually soften and melt FeOOH nanopillar, leading to tortuosity and agglomeration via grain-boundary motion and oriented attachment, subsequently making the derived Fe2O3 collapse and tightly join together. Instead, with RD route, the FeOOH nanopillar undergoes immediate and rapid dehydration, and quickly transform into Fe2O3. With a high melting point of 1565 oC, the Fe2O3 nanopillars thus preserve the robust pillar arrays of as-grown FeOOH. The possible formation mechanism of nanopores in RD-Fe2O3 may originate from the fast expulsion of water from the dehydration of FeOOH during the initial annealing treatment. With further increase in temperature, the forming micropores are coalesced and transformed into nanoporous via surface diffusion-coalescence mechanism.30, 31 For investigating the optimum RD treatment temperature, the photocurrent density of Fe2O3 photoanodes obtained at different RD temperatures from 100 oC to 300 oC are measured (Figure S4). The results show that the photocurrent density 7



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gradually increases with rising of RD temperatures, and reaches maximum at above 250 oC. It indicates that 250 oC is a critical temperature point to reduce structural collapse and agglomeration or coalescence of Fe2O3 nanopillars after annealing treatment. As displayed in Figure S1(c, f), directly annealed sample at 250 oC, has formed uniform nanopores and well preserves nanopillar morphology at the temperature point of 250 oC, compared with disorderly dense and partly tightening morphology of slowly heated-up from room temperature to 250 oC sample [Figure S1(b, e)]. Figure 4a shows the photocurrent density-potential curves of C-Fe2O3 and RD-Fe2O3 in dark and under front and back illuminations (100 mW cm-2, AM 1.5G). The RD-Fe2O3 exhibits an enhanced photocurrent density as high as 2.0 mA cm-2 at 1.23 V vs. RHE without using any co-catalysts, about three times higher than that of C-Fe2O3 (0.75 mA cm-2 at 1.23 V vs. RHE). A plateau of about 1.48 mA cm-2 for C-Fe2O3 and 3.50 mA cm-2 for RD-Fe2O3 at 1.71 V vs. RHE are observed, which is consistent

with

light-chopped

photocurrent-potential

curves

(Figure

S5a).

Photocurrent transients associated with recombination of charge carriers disappear at higher potentials, indicating better injection and low recombination of holes on the photoanode surface. For the C-Fe2O3, lower photocurrent obtained under front illumination measurement than back illumination. As a contrast, the photocurrent difference on RD-Fe2O3 was not obvious both under front and back illuminations. It demonstrates that existence of the nanopore shorten the hole transport distance from bulk material to electrode/electrolyte interface. The RD-Fe2O3 also possesses excellent stability, exhibiting no obvious decay for more than 5 h in 1 M aqueous NaOH under AM 1.5G illumination (Figure S5b). Moreover, a catholically shifted onset potential of about 80 mV is observed on the RD-Fe2O3 from 1.03 to 0.95 V vs. RHE relative to the C-Fe2O3, indicating the back reaction is intensely suppressed on RD-Fe2O3 due to its good contact interfacial with FTO glass substrate (Figure S6). In addition, the RD-Fe2O3 photoanode displays significantly enhanced incident photon-to-current conversion efficiency compared with that of C-Fe2O3 over the entire measured 8


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spectrum (Figure 4b), 40.2% at 370 nm for RD-Fe2O3, doubling that of C-Fe2O3 (20.9% at 370 nm) and IPCE-integrated photocurrent density is detected 1.91 mA cm-2 at 1.23 V vs. RHE (Figure S7). This is agreement with the J-V measurements within a margin of error. The enhanced PEC performance on RD-Fe2O3 photoanode obviously results from its micro-structure advantages. Firstly, UV-vis absorption spectroscopy illustrates that absorption intensity of RD-Fe2O3 is apparently improved as compared to C-Fe2O3 due to its long nanopillars (Figure 5a), which could excite higher charge carrier density for PEC water oxidation. It should be mentioned that the pores can not multi-reflect light for the enhancement of light absorption and less contribute to improvement of the photoelectrichemical performance. The reason is small pore size (~20 nm) of the porous structure, much shorter than the incident wavelength of 200-800 nm. Secondly, the Fe2O3 nanopillars with porous structure, offer short transport distance for photogenerated holes to reach electrode/electrolyte interface, benefitting for more holes to drive the water oxidation reaction, as schematically illustrated in Figure 6. The good PEC performance of RD-Fe2O3 also originates from its excellent hydrophilicity with much small contact angle of 5.7°, relative to 40.7° for C-Fe2O3. The good hydrophilicity property of RD-Fe2O3 could accelerate peripheral electrolyte to easily penetrate on the interface between the electrode-electrolyte for supplying the electrolyte depletion with water splitting process (Figure S8), showing better kinetics of reaction and establishing more semiconductor-electrolyte interface for charge separation. Thirdly, preservation of nanopillar arrays against structural collapse and agglomeration or coalescence decreases charge carrier recombination and enhance the holes transfer properties. J-V curves of C-Fe2O3 and RD-Fe2O3 are tested to explain the conclusion (Figure 5b). Compared with C-Fe2O3, higher current density is detected on RD-Fe2O3 at same bias voltage, indicating better hole conductivity. The electrochemical impedance spectroscopy (EIS) confirms the reduced charge recombination on the RD-Fe2O3. The Nyquist curves are fitted using circuit elements consisting of one resistor and two RC (resistance and capacitance) 9



circuits in parallel (Figure S9a). Bulk Fe2O3 resistance (Rbulk) decreases to 240.8 Ω cm-2 for RD-Fe2O3, relative to 404 Ω cm-2 for C-Fe2O3 (Figure S9b). The solid-fluid interface resistance (RSLI) decreases remarkably from 87.05 Ω cm-2 for C-Fe2O3 to 3.12 Ω cm-2 for RD-Fe2O3, hinting more holes are migrated onto RD-Fe2O3 surface for water oxidation reaction. To examine the charge transfer kinetics which results from preservation of nanopillar arrays, the transient absorption spectroscopy and transient PL decay are measured. The RD-Fe2O3 shows relatively slower decay than C-Fe2O3 in both the 0-5 ps windows (Figure 5c), indicating the formation of longer lived hole which survived from initial recombination. Both C- and RD-Fe2O3 films were spin-coated with a compact CH3NH3PbI3 layer as a hole-transport material to test photoluminescence lifetime as alone Fe2O3 generally exhibit very weak photoluminescence due to serious nonradiative recombination (Figure 5d). The decay spectrum is fitted with a double-exponential function of time t: F(t) = Aiet/ti; i = 1, 2, where Ai is a prefactor and ti is the time constant. The average minority carrier lifetime (tave) is derived to be 4.0 ns for C-Fe2O3 and 12.1 ns for RD-Fe2O3 and with Ai and ti according to the equation tave = Aiti2/Aiti; i= 1, 2. The increase in tave further demonstrates that the robust nanopillar morphology facilitates charge transfer of RD-Fe2O3, restraining the charge recombination. Comparison of PEC performance and morphology of different metal oxide photoanodes with the present Fe2O3 was listed in Table S1.

4. Conclusions In summary, a facile so-called RD strategy is able to fabricate robust Fe2O3 porous nanopillars through quasi-topotactic transformation of FeOOH nanorods. The RD process not only retain the FeOOH precursor, alleviate structural collapse and agglomeration or coalescence of the resulting Fe2O3 nanopillars, but also generate beneficial porous structure. The RD-Fe2O3 photoanode exhibits a photocurrent density as high as 2.0 mA cm-2 at 1.23 V vs. RHE without using any co-catalysts, which is about 3-time enhancement over C-Fe2O3. It believes that the present study may open 10


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up new opportunities for the design and fabrication of high performance Fe2O3 electrodes for PEC reactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:xxxxx SEM and TEM images; XRD pattern; XPS spectra; Current density-voltage curves; IPCE spectra; EIS

Author information Corresponding Author *

E-mail address: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by 973 Programs (No. 2014CB239302 and 2013CB632404), NSF of China (No. 21773114, 21473091, 21603183, 41702037), NSF of Jiangsu Province (No. BK20171246, BK20160412, and BK20130425), and Jiangsu Postdoctoral Science Foundation (1601062B).

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Mosaic Structure in Hematite Derived from Goethite. J. Solid State Chem. 1979, 29, 137-150.

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Figure Captions: Figure 1 A schematic illustration of the preparation of C-Fe2O3 and RD-Fe2O3. Figure 2 (a-c) SEM images of FeOOH. (d-f) SEM images of C-Fe2O3. (g-i) SEM images of RD-Fe2O3. Figure 3 (a, b and inset of b) TEM images and SAED pattern for C-Fe2O3, respectively. (c and d) TEM images, (e) high-resolution TEM image, (f) SAED pattern, (g-i) TEM and EDS mapping images of O and Fe for RD-Fe2O3, respectively. Figure 4 (a) Current density-voltage curves of C-Fe2O3 and RD-Fe2O3 collected at 10 mV s-1 in a 1.0 M KOH aqueous electrolyte under AM 1.5G illumination and in dark. The solid and dashed lines represent the data collected under back (solid lines) and front (dashed lines) illuminations, respectively. (b) IPCE spectra of C-Fe2O3 and RD-Fe2O3 collected at 1.23 V vs. RHE. Figure 5 (a) UV-vis absorption spectra and the inset is optical photograph of the C-Fe2O3 and RD-Fe2O3, respectively. (b) J-V curves for the hole-only devices fitted with the Mott-Gurney law. The insert is the sketch of hole-only device configuration. (c) Normalized transient absorption vs. time curves of C-Fe2O3 and RD-Fe2O3 in air atmosphere, λex = 350 nm, at 575 nm. (d) Normalized time-resolved PL decay curves of C-Fe2O3 and RD-Fe2O3. Figure 6 Possible sketch of the transfer of photogenerated charge carriers in C-Fe2O3 and RD-Fe2O3.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Table of Contents (TOC)

A facile rapid dehydration strategy (RD) is able to fabricate robust Fe2O3 porous nanopillars through quasi-topotactic transformation of FeOOH nanorods, avoiding collapse, shrinking, and coalescence. The so-called RD process effectively boosts the photoelectrochemical performance of Fe2O3 photoanode.

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Quasi-Topotactic Transformation of FeOOH ... - ACS Publications

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