Controlled Growth of Ferrihydrite Branched Nanosheet Arrays and

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Controlled Growth of Ferrihydrite Branched Nanosheet Arrays and Their Transformation to Hematite Nanosheet Arrays for Photoelectrochemical Water Splitting Mei Ji, Jinguang Cai, Yurong Ma, and Limin Qi* Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The morphology engineering represents an alternative route toward efficient hematite photoanodes for photoelectrochemical (PEC) water splitting without changing the chemical composition. In this work, a facile and mild solvothermal synthesis of unique ferrihydrite branched nanosheet arrays vertically aligned on FTO substrate was achieved at around 100 °C. The hierarchical branched ferrihydrite nanosheet arrays consisted of tiny branches up to 40 nm in length grown almost vertically on stem nanosheets ∼10 nm in thickness. Moreover, the variation of the morphology of the ferrihydrite nanostructures from bare nanosheet arrays through branched nanosheet arrays to dense branched structures can be readily achieved through the regulation of the reaction time and temperature. The obtained ferrihydrite branched nanosheet arrays can be in situ transformed into α-Fe2O3 nanosheet arrays with small surface protrusions upon annealing at 550 °C. After a simple postgrowth Ti-doping process, the resulting Ti-doped α-Fe2O3 nanosheet arrays showed a good PEC performance for water splitting with a photocurrent density of 1.79 mA/cm2 at 1.6 V vs RHE under AM 1.5G illumination (100 mW/cm2). In contrast, the Ti-doped irregular aggregates of the α-Fe2O3 nanograins transformed from dense ferrihydrite branched structures exhibited a much lower photocurrent density (0.41 mA/cm2 at 1.6 V vs RHE), demonstrating the important influence of the morphology of α-Fe2O3 photoanodes on the PEC performance. KEYWORDS: photoelectrochemical water splitting, hematite, ferrihydrite, nanosheet arrays, morphology engineering

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INTRODUCTION Artificial photosynthesis in the form of photoelectrochemical (PEC) water splitting is a promising strategy for converting solar energy to chemical fuels such as hydrogen in a sustainable and cost-efficient manner.1−3 Among all the available semiconductor photocatalysts, hematite (α-Fe2O3) has emerged as an attractive candidate for PEC photoanode materials due to its favorable bandgap (2.0−2.2 eV) for broad visible light absorption, high theoretical solar-to-hydrogen conversion efficiency (16.8%), natural abundance, low cost, nontoxcity, and excellent chemical stability.4−6 However, several intrinsic properties of hematite restrict its practical applications in PEC water splitting, such as short lifetime of photogenerated charge carriers (98.0%), sodium hydroxide (>98.0%), and acetylacetone (>99.0%) were obtained from Xilong Chemical Co., Ltd. Ethanol (>99.7%) was obtained from Beijing Chemical Works, and tetrabutyl titanate (>98%) was obtained from Beijing Yili Fine Chemicals Co., Ltd. Synthesis. Ferrihydrite nanostructure arrays were solvothermally grown on FTO coated glass substrates. In a typical synthesis, FTOcoated glass slides of 1.25 cm × 5 cm were first cleaned by sonication in deionized water, ethanol, acetone and ethanol separately for 20 min. Fe(acac)3 (0.4 mmol) was dissolved in 5 mL ethanol, which was then mixed with a 10 mL of aqueous solution containing NaNO3 and NaF, resulting in a translucent suspension. The final concentration of the NaNO3 and NaF was 1 M and 15 mM, respectively. The suspension was transferred into a 25 mL Teflon-lined autoclave, and FTO glass slides were placed with the FTO side facing the wall of the autoclave slantways. The autoclave was placed at a certain temperature (typically, 100 °C) for 5 h. After the reaction, a uniform yellow thin film of ferric oxyhydroxide formed on the FTO substrate, which was thoroughly rinsed with deionized water and ethanol, and then dried in an oven at 50 °C overnight. The effects of the temperature, the reaction time, the addition of acetylacetone, the NaF and NaNO3 concentrations, and the ethanol/water volume ratio on the ferrihydrite products were investigated, respectively. The obtained ferrihydrite nanostructure arrays were annealed in air at 550 °C for 2 h to achieve their transformation to a red thin film of α-Fe2O3 nanostructure arrays. For the applications in PEC water splitting, a simple postgrowth Tidoping process was employed following the literature11 with some modification. Briefly, the as-prepared α-Fe2O3 nanostructure arrays were treated by a simple spin-coating method with a 15 μL ethanol solution containing 50 mM tetrabutyl titanate at a rate of 2000 rpm for 30 s, which was followed by annealing in air at 750 °C for 10 min. For comparison, undoped α-Fe2O3 nanosheet arrays were prepared by annealing the as-prepared α-Fe2O3 nanosheet arrays in air at 750 °C for 10 min without deposition of tetrabutyl titanate. After the annealing, the samples were cooled down to room temperature in the furnace naturally. Characterizations. The samples were characterized by scanning electron microscopy (SEM, Hitachi S4800, 10 kV), transmission B

DOI: 10.1021/acsami.5b08116 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces electron microscopy (TEM), high-resolution TEM (HRTEM, FEI Tecnai F30, 300 kV), powder X-ray diffraction (XRD, Rigaku Dmax2000, Cu Kα radiation), and Fourier transform IR spectroscopy (FTIR, Nicolet Magna-IR 750). Photoelectrochemical Measurements. Photoelectrochemical measurements were performed using a CHI 660C working station (CH Instrument, Inc.) in a three-electrode electrochemical cell, with an α-Fe2O3 photoanode as the working electrode, a Pt electrode as a counter electrode and an Ag/AgCl electrode as a reference electrode. The electrolyte was an aqueous solution with 1 M NaOH (pH 13.6). A 300 W Xe lamp coupled with an AM 1.5G filter was used as the white light source. The light power density at the sample surface was measured to be 100 mW/cm2. The α-Fe2O3 photoanodes were covered by nonconductive parafilm for a working area of 0.25 cm2. The photocurrent density was measured with a sweep rate of 50 mV/s, and the illumination through the FTO side of the samples. The electrochemical impedance spectra (EIS) were recorded under illumination at an applied potential of 0.23 V vs Ag/AgCl and a frequency range of 10−100 kHz with an amplitude of 10 mV. The Mott−Schottky analysis was carried out with a potential frequency of 1 kHz under dark condition. The measured potential vs Ag/AgCl reference electrode was converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: ERHE = EAg/AgCl + 0.059 pH + EoAg/AgCl, where ERHE is the converted potential vs RHE, EoAg/AgCl = 0.1976 V at 25 °C, and EAg/AgCl is the experimentally measured potential against the Ag/AgCl reference electrode.

vertically aligned nanosheets less than 100 nm in thickness and ∼500 nm in width grown on the FTO substrate. An enlarged SEM image shown in Figure 1b suggests that there are many tiny branches up to 40 nm in length grown on the stem nanosheets with a thickness about 10 nm. The TEM image shown in Figure 1c confirms the hierarchical branched nanosheet morphology with the branches ∼5 nm in diameter grown almost vertically on the stem nanosheet. The XRD pattern of the ferrihydrite branched nanosheet arrays on FTO is presented in Figure 2, which shows only the reflections arising

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RESULTS AND DISCUSSION Figure 1 shows representative electron microscopy characterizations of the ferrihydrite branched nanosheet arrays obtained after solvothermal growth on FTO at 100 °C for 5 h. As shown in Figure 1a, the thin film on FTO apparently consists of

Figure 2. XRD patterns of ferrihydrite powders, ferrihydrite branched nanosheet arrays on FTO, and α-Fe2O3 nanosheet arrays on FTO. The standard diffraction patterns of hematite (JCPDS 33−0664) and ferrihydrite (JCPDS 46−1315) are also displayed. Black triangles denote the diffraction peaks attributed to the FTO substrate. The ferrihydrite powders were obtained by scraping ferrihydrite branched nanosheet arrays from the FTO substrate.

from the FTO substrate because of the small amount of the thin film on FTO. When the thin film was carefully scraped from the FTO substrate, the obtained powder sample shows a weak but well-resolved diffraction peak at d ≈ 0.313 nm, which can be indexed to the characteristic (003) reflection of ferrihydrite (JCPDS card number: 46−1315), indicating that the branched nanosheet arrays are made of ferrihydrite crystals. It may be noted that the ferrihydrite (002) and (005) reflections, which also have strong diffraction intensity in the standard card, can not be well resolved possibly owing to a slight departure of the poorly crystallized phase from the perfect crystal structure of ferrihydrite (JCPDS 46−1315). As shown in Figure 1d, the selected-area electron diffraction (SAED) pattern of the branches consisting of tiny nanorods shows broadened spots and intermittent rings, which can be well ascribed to the ferrihydrite (110), (112), (005), (114), (115) planes, confirming that the branches are ferrihydrite nanocrystals. Figure 1e shows a typical HRTEM image of the stem nanosheet of a branched nanosheet vertically standing on the copper grid. It exhibits clear lattice fringes with spacings of 0.31 and 0.25 nm, which can be indexed to the (003) and (110) planes of ferrihydrite, indicating that the stem nanosheet is a single-crystalline nanosheets with exposed (001) basal planes. The existence of some apparent defects inside the stem nanosheet suggests that it is poorly crystallized. The HRTEM image of the side branches shows clear lattice fringes with a spacing of 0.25 nm corresponding to the (110) plane of ferrihydrite (Figure 1f), suggesting that the branches consist of

Figure 1. (a, b) SEM images of ferrihydrite branched nanosheet arrays grown at 100 °C for 5 h. Inset shows a side-view SEM image. (c) TEM image of isolated precursor branched nanosheet. (d) SAED pattern of side branches corresponding to the circled area in panel c, HRTEM images of (e) stem nanosheet and (f) side branches of precursor branched nanosheets. C


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calcination at 550 °C, resulting in hematite nanosheets with coarse surfaces and an overall thickness around 40−50 nm. The TEM image of an individual nanosheet with a semicircular configuration suggests that the nanosheet consists of nanosized grains, in good agreement with the SEM observation. The corresponding SAED pattern (Figure 3d) shows many scattered spots with d-spacing of 0.270, 0.252, 0.220, and 0.145 nm, which can be indexed to the α-Fe2O3 (104), (110), (113), and (300) planes, respectively, indicating a polycrystalline nanosheet. The HRTEM image presented in Figure 3e shows nanosized grains exhibiting lattice fringes with spacings of 0.15 nm, 0.22 nm, 0.25 and 0.27 nm, which is consistent with the SAED pattern. These results suggest that the coarse αFe2O3 nanosheets transformed from the branched ferrihydrite nanosheets are polycrystalline nanosheets consisting of randomly oriented hematite nanocrystals. The effects of the reaction temperature on the morphology of the ferrihydrite nanostructure arrays were investigated. When the reaction temperature was decreased from 100 to 90 °C, the products consist of nanosheets with an average width of ∼400 nm and sparse, short branches grown on the side faces (Figure 4a). After calcination at 550 °C, the precursor nanosheet arrays

ferrihydrite nanorods, in good agreement with the SAED result. Therefore, it may be concluded that the hierarchical thin film grown on FTO is made of ferrihydrite branched nanosheet arrays consisting of numerous nanorods grown on side faces of vertical (001)-oriented stem nanosheets. It may be noted that among synthetic ferrihydrite samples, two-line ferrihydrite that shows two very broad maxima at about 2.54 and 1.5 Å in the XRD pattern and 6-line ferrihydrite that shows six broad but distinct peaks between 2.56 and 1.48 Å in the XRD pattern are the most frequently observed.49,56 The synthetic ferrihydrite (JCPDS Card No. 46−1315) with a characteristic X-ray diffraction peak at 2θ value around 28.5° (d ≈ 3.13 Å) was just occasionally observed.57,58 In the current situation, the formation of the less common ferrihydrite phase might be attributed to the unique synthetic system involving the solvothermal hydrolysis of Fe(acac)3 in the mixture of ethanol and water. The transformation of the as-prepared ferrihydrite branched nanosheet arrays into α-Fe2O3 nanosheet arrays was achieved by annealing at 550 °C in air for 2h. Figure 2 presents the XRD pattern of the resultant α-Fe2O3 nanosheet arrays, which exhibits the characteristic (104) and (110) reflections of hexagonal α-Fe2O3 phase (JCPDS No. 33−0664) in addition to the reflections arising from the FTO substrate, indicating a conversion from ferrihydrite to hematite crystals. Figure 3a

Figure 4. SEM images of (a) ferrihydrite nanosheet arrays grown at 90 °C and (b) the transformed α-Fe2O3 nanosheet arrays. (c) SEM, (d) TEM, and (e) HRTEM images of ferrihydrite branched structures obtained grown at 120 °C and (f) SEM image of the transformed αFe2O3 nanostructure arrays.

Figure 3. (a, b) SEM images of α-Fe2O3 nanosheet arrays. Inset shows a side-view SEM image. (c) TEM image, (d) SAED pattern, and (e) HRTEM image of an individual α-Fe2O3 nanosheet.

converted into α-Fe2O3 nanosheet arrays with slightly rough surfaces (Figure 4b). If the reaction temperature was increased from 100 to 120 °C, unique branched structures consisting of two layers of dense branches ∼90 nm in length separated by a thin gap were obtained (Figure 4c). The TEM image shown in Figure 4d confirmed the existence of a thin layer of void space in the middle of the branched structure, indicating the disappearance of the stem nanosheet. The HRTEM image of the branches shows clear lattice fringes with a spacing of 0.25

presents a typical SEM image of the obtained α-Fe2O3 nanosheet arrays, which shows that the nanosheets (∼500 nm in width and ∼350 nm in height) vertically aligned on the FTO substrate essentially preserved the nanosheet array structure of the ferrihydrite precursor thin film. However, the enlarged SEM image shown in Figure 3b suggests that the side branches grown on the stem nanosheets coagulated into protuberances on the surface of the nanosheets upon D


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acac. When NaF was absent, branched structures were also obtained but the stem wires rather than stem sheets were formed in the center (Figure S1a). When the NaF concentration was increased to 5 mM, branched nanosheets consisting of side branches grown on discontinuous stem nanosheets (Figure S1b). Well-defined branched nanosheet arrays were obtained when the NaF concentration was increased to 15 mM, as shown in Figure 1. Similar branched nanosheet arrays with well-resolved stem nanosheets were produced when the NaF concentration was further increased to 25 mM (Figure S1c). This result indicates that the presence of F− ions is essential to the formation of the ferrihydrite nanosheet arrays. Considering that acetylacetonate group would be gradually released with the hydrolysis of Fe(acac)3, the subsequent growth of branches on the side faces of the vertical nanosheets could be attributed to the presence of excess acetylacetonate group in the solution. It may be reasonably speculated that there exists competitive adsorption on the growing ferrihydrite crystals between F− ions and acac group, and the presence of F− ions favored the preferential growth of the stem nanosheets whereas the gradual release of acetylacetonate group promoted the subsequent growth of the side branches. Such a competitive effect between F− ions and acac group was preliminarily revealed by the FTIR spectra of the ferrihydrite nanosheet arrays obtained with different NaF concentrations (Figure S2). The product obtained without NaF shows well-resolved IR peaks at 1582 cm−1 for ν(CO), 1523 cm−1 for ν(CC), 1367 cm−1 for δs(C−H), 1275 cm−1 for δ(CC−H), which are characteristic of the acac group, indicating the presence of adsorbed acac group.44 With increase NaF concentration, the IR peaks are gradually weakened, suggesting that the amount of adsorbed acac group is decreased with increasing the F− ion concentration in the solution, probably because of the competitive effect. As a further demonstration, acetylacetone, which has a strong coordination ability similar to acac group, was introduced to the reaction system. As expected, the addition of acetylacetone considerably enhanced the growth of the branches and inhibited the formation of complete nanosheets (Figure S3). It may be pointed out that an appropriate ethanol/water volume ratio and the presence of a high NaNO3 concentration are also necessary for the formation of the unique ferrihydrite branched nanosheet arrays. Both decrease in the ethanol/water volume ratio and the NaNO3 concentration led to the formation of irregular particles instead of regular branched nanosheet arrays (Figures S4 and S5). This may be rationalized by considering that a relatively low water content would bring about a relatively mild hydrolysis reaction, whereas a high ionic strength would be favorable for the heterogeneous nucleation and growth of nanostructure arrays on substrates rather than uncontrolled precipitation from solution.61 On the basis of the above experimental observations, a possible mechanism of the growth of the hierarchical feerihydrite nanostructure arrays on FTO and their transformation to hematite nanostructure arrays was tentatively proposed (Figure 6). At the early stage of the hydrolysis of Fe(acac)3 in mixed ethanol−water solvents in the presence of NaF and NaNO3, the nucleation process of ferrihydrite occurred on the FTO substrate because of the relatively low supersaturation in the solution owing to the mild hydrolysis reaction. Meanwhile, the preferential adsorption of F− ions on the growing ferrihydrite crystals resulted in vertically aligned arrays of poorly crystallized ferrihydrite nanosheets. When the

nm corresponding to the (110) planes of ferrihydrite (Figure 4e), suggesting that the branches are well crystallized ferrihydrite nanorods. After annealing at 550 °C, the coalescence of the branched structures resulted in the formation of irregular aggregates of α-Fe2O3 nanograins (Figure 4f). A time-dependent investigation was carried out by examining the ferrihydrite nanostructure arrays grown at 100 °C on FTO at different growth stages. At a reaction time of 2 h, nearly bare nanosheets with a width of 150−300 nm were grown on the surface of the substrate (Figure 5a). When the reaction time

Figure 5. SEM images of ferrihydrite nanostructure arrays grown at 100 °C for different times: (a) 2, (b) 4, (c) 9, and (d) 24 h.

was increased to 4 h, the nanosheets became larger with an average width of 350 nm and there were short branches sparsely grown on the surface (Figure 5b). At a reaction time of 5 h, the branches on both faces of the stem nanosheets grew up to ∼45 nm in length whereas the nanosheet width grew up to ∼500 nm, as shown in Figure 1. When the reaction time was extended to 9 h, the side branches became thicker with a length of ∼85 nm (Figure 5c). After the reaction time was further prolonged to 24 h, the side branches evolved into two layers of dense branches with a length of ∼105 nm, leaving a thin void space in the middle (Figure 5d). It is noteworthy that the variation of the ferrihydrite morphology from bare nanosheet arrays through branched nanosheet arrays to branched structures without stem nanosheets with reaction time is very similar to the variation trend with increasing reaction temperature from 90 to 120 °C, whereas the reaction time was kept as 5 h. This result indicates that effects of raising reaction temperature are similar to those of prolonging reaction time, namely, the reaction extent would increase by increasing either the reaction time or the reaction temperature. To shed light on the growth mechanism of the unusual ferrihydrite branched nanosheet arrays, the effects of the composition conditions of the synthesis system on the ferrihydrite products were investigated. Since both F − anions59,60 and the acetylacetonate (acac) group44 have strong interactions with Fe(III) species and ferric (oxy)hydroxides, particular attention was paid to the effects of the NaF concentration and the addition of acetylacetone. The typical ferrihydrite branched nanosheet arrays were obtained by the hydrolysis of Fe(acac)3 in mixed ethanol−water solvents in the presence of 15 mM NaF and 1 M NaNO3 without addition of E


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Figure 6. Schematic of the growth process of ferrihydrite nanostructure arrays on FTO substrate and their transformation to α-Fe2O3 nanostructure arrays.

reaction time was increased, the gradual release of acac group promoted the growth of tiny branches on side faces of the stem nanosheets because of the specific coordination ability of the acac group, leading to the formation of the hierarchical ferrihydrite branched nanosheet arrays. If the reaction was further prolonged, the growth of the side branches was accelerated owing to the increased amount of acac group in the solution and finally they evolved into two layers of dense, well crystallized branches. Meanwhile, the initial stem nanosheets were gradually etched possibly by the acac groups with strong coordination ability, leading to the formation of branched ferrihydrite nanostructures with a thin layer of void space in the middle. Essentially, raising the reaction temperature and prolonging the reaction time would have similar effects on the evolution of the ferrihydrite nanostructure arrays. Calcination at 550 °C would lead to the transformation of ferrihydrite to hematite and the simultaneous coalescence of the branches. Accordingly, the ferrihydrite precursor nanostructure arrays obtained at different growth stages would convert into the corresponding α-Fe2O3 nanostructure arrays in situ on the FTO substrate. Nevertheless, further detailed experiments are still required to elucidate the formation mechanism of the unusual ferrihydrite nanostructure arrays at the molecular scale. The obtained α-Fe2O3 nanosheet arrays on FTO can be used as a good photoanode for photoelectrochemical (PEC) water splitting. For the application in PEC water oxidation, a simple postgrowth Ti-doping of the as-prepared α-Fe2O3 nanostructure arrays was performed by spin-coating of a tetrabutyl titanate solution and subsequent annealing in air at 750 °C.11 After the annealing, the morphology of the α-Fe2O3 nanosheet arrays was essentially unchanged although the nanosheet surfaces became somewhat smoother (Figure S6). Figure 7a depicts the photocurrent density-potential (I−V) curves of the undoped α-Fe2O3 nanosheet array (NSA) and Ti-doped αFe2O3 NSA electrodes, both of which underwent annealing at 750 °C, in the dark and under AM 1.5G illumination (100 mW/cm2) in a 1 M NaOH electrolyte solution. Under dark environments, the photocurrent density is essentially zero for both the undoped and Ti-doped samples at an applied bias lower than 1.6 V vs RHE. Under AM 1.5G illumination, the photocurrent density for the undoped nanosheet array electrode is 0.23 mA/cm2 at 1.23 V vs RHE and 0.53 mA/ cm2 at 1.6 V vs RHE. It may be noted that the original α-Fe2O3 nanosheet arrays transformed from ferrihydrite nanoarrays at 550 °C without annealing at 750 °C showed only poor PEC performance (∼0.016 mA/cm2 at 1.23 V vs RHE), which could be partly attributed to the presence of a large number of highangle grain boundaries that can be eliminated upon heating at

Figure 7. (a) Photocurrent density-potential (I−V) curves for PEC water oxidation of α-Fe2O3 nanosheet array (α-Fe2O3 NSA) and Tidoped α-Fe2O3 nanosheet array (α-Fe2O3 NSA-Ti) electrodes. (b) Transient photocurrent density-time curves of α-Fe2O3 NSA-Ti at applied potentials of 1.23 and 1.60 V vs RHE measured under AM 1.5G illumination in a 1 M NaOH electrolyte solution. (c) Mott− Schottky plots of α-Fe2O3 NSA and α-Fe2O3 NSA-Ti. (d) EIS Nyquist plots of α-Fe2O3 NSA and α-Fe2O3 NSA-Ti measured at an applied potential of 1.23 V vs RHE.

temperatures near 800 °C.35 After Ti-doping, the photocurrent density increases to 0.77 mA/cm2 at 1.23 V vs RHE and 1.79 mA/cm2 at 1.6 V vs RHE, respectively. The PEC performance is much higher than that for the Ti-doped α-Fe2O3 film obtained by a hydrothermal method (0.15 mA/cm2 at 1.23 V vs RHE),62 but considerably lower than that for the single crystal Ge-doped nanosheet arrays (1.4 mA/cm2 at 1.23 V vs RHE).47 It is worth pointing out that the PEC performance of the Tidoped α-Fe2O3 nanosheet arrays was dependent on the doping process. If the ferrihydrite branched nanosheet arrays without calcination at 550 °C were directly used as the precursor for Tidoping under the similar conditions (namely, spin-coating with tetrabutyl titanate followed by annealing at 750 °C for 10 min), relatively lower photocurrent densities (0.52 mA/cm2 at 1.23 V vs RHE and 0.86 mA/cm2 at 1.6 V vs RHE), which are considerably higher than those of the undoped α-Fe2O3 nanosheet arrays, were achieved (Figure S7). This result indicates that the Ti doping also occurred in the doping process using the ferrihydrite branched nanosheets as the precursor, but the resultant Ti-doped α-Fe2O3 nanosheet arrays could have a lower Ti-doping extent and a poorer crystallinity (or more grain boundaries) compared with the Ti-doped α-Fe2O3 nanosheet arrays obtained using the transformed α-Fe2O3 nanosheets for the Ti-doping. To further examine the photoresponse, we measured the transient photocurrents of the Ti-doped α-Fe2O3 NSA (αFe2O3 NSA-Ti) electrode under chopped light at applied potentials of 1.23 V vs RHE and 1.6 V vs RHE (Figure 7b). There is a transient anodic photocurrent peak when the chopped light is turn on, which reflects the accumulation of holes at the electrode−electrolyte interface. The transient cathodic photocurrent peak when the chopped light is turn off, represents the back reaction of electrons from the conduction band with the accumulated holes.63 A rapid and reversible photocurrent responses can be observed for each switching on and off, and the photocurrent density just has a slight decrease F


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ACS Applied Materials & Interfaces from 0.70 mA/cm2 to 0.65 mA/cm2 at 1.23 V vs RHE and from 1.63 mA/cm2 to 1.59 mA/cm2 at 1.6 V vs RHE after 400 s. The time-dependent photocurrent density of the α-Fe2O3 NSA-Ti electrode shows that there was a slow decrease of the photocurrent density at 1.6 V vs RHE with time and a photocurrent density of 1.52 mA/cm2 was retained after 2 h of AM 1.5G illumination (Figure S8), indicating a good stability of the photoanode. To understand the improvement in photocurrent density upon Ti-doping, we measured the electrochemical impedance of the undoped and Ti-doped α-Fe2O3 samples in the dark at a frequency of 1 kHz, and the calculated Mott−Schottky plots are shown in Figure 7c. It is known that Mott−Schottky plots can give the information about the donor concentration (Nd) and the flat-band potential (VFB) through the Mott−Schottky equation 1 2 ⎛ kT ⎞ = ⎜V − VFB − ⎟ 2 qεε0Nd ⎝ q ⎠ C

(1)

Where C is the capacitance per unit area, q is the elementary charge, ε is the dielectric constant of hematite (80),64 ε0 is the permittivity of vacuum, Nd is the donor density, V is the electrode applied potential, k is the Boltzmann constant, and T is the absolute temperature. Both samples show a positive slope in the Mott−Schottky plots, indicating that they are n-type semiconductors. The donor density can be calculated as ⎡ 1 2 ⎢ d C2 Nd = qεε0 ⎢⎢ dV ⎣

Figure 8. (a) Photocurrent density-potential (I−V) curves for PEC water oxidation and (b) EIS Nyquist plots at an applied potential of 1.23 V vs RHE of Ti-doped α-Fe2O3 nanostructure arrays obtained at different growth temperatures measured under AM 1.5G illumination in a 1 M NaOH electrolyte solution.

( ) ⎤⎥

−1

⎥ ⎥⎦

(2)

aggregates of α-Fe2O3 nanograins obtained at 120 °C exhibit the lowest photocurrent density, namely, 0.41 mA/cm2 at 1.6 V vs RHE. The EIS Nyquist plots of the three samples are shown in Figure 8b, which suggests that the charge transfer resistance of the irregular aggregates obtained at 120 °C was significantly larger than those of the nanosheet arrays obtained at 90 and 100 °C. The charge transfer resistance of the nanosheet arrays obtained at 90 °C is somewhat larger than that of the nanosheet arrays obtained at 100 °C, which could partially account for the PEC performance of the three samples. These results can be rationalized by considering that the vertical aligned 2D nanosheets have direct electron transport pathways and large electrode−electrolyte interface, which would benefit the electron transport in the photoanode and facilitate oxidation of water at the interface. As for the irregular aggregates of nanograins, the electron transport would be considerably hindered because the photogenerated electrons would pass much more interfaces between grains and the recombination would happen more easily. Compared with the nanosheet arrays obtained at 90 °C, there is a small increase in the photocurrent density for the nanosheet arrays obtained at 100 °C, which could be attributed to the increase in the overall amount of the deposited α-Fe2O3 on FTO because of the slight increase in the thickness and width of the nanosheets as well as the increase in the surface area because of the presence of small surface protuberances. It may be noted that the irregular aggregates obtained at 120 °C showed a much lower photocurrent density despite a further increase in the amount of the deposited α-Fe2O3 because of a morphology change from arrayed nanosheets to irregular aggregates. These results demonstrate that the morphology of α-Fe2O3 photoanodes plays a key role in determining their PEC performance, and the

As a result, the donor density Nd was calculated to be 2.97 × 1019 cm−3 and 4.55 × 1020 cm−3 for the undoped α-Fe2O3 and the Ti-doped α-Fe2O3, respectively. The increase in donor density upon Ti-doping improves the conductivity, and thereby the collection efficiency of photogenerated electrions.9 The measured donor density for the Ti-doped α-Fe2O3 nanosheet array is comparable to those of the Ti-doped α-Fe2O3 photoanodes reported previously.11 The electrochemical impedance spectroscopy (EIS) analysis has been frequently used to analyze the charge transfer process at the semiconductor-electrolyte.25 Figure 7d shows the EIS spectra at 1.23 V vs RHE under AM 1.5 G illumination for the undoped and Ti-doped α-Fe2O3 samples. The low frequency response can be assigned to the semiconductor-electrolyte charge transfer.65,66 Obviously, the radius of the low frequency response becomes smaller after Ti-doping implies considerable enhancement of the interface charge transfer rate and hence effective suppressing surface recombination of photogenerated hole and electron.20 Because the PEC performance of the α-Fe2O3 photoanode could be largely influenced by the morphology of the α-Fe2O3 nanostructures, the Ti-doped α-Fe2O3 nanostructure arrays prepared from ferrihydrite nanostructure arrays grown at different temperatures were employed for the PEC measurements. The photocurrent density-potential curves shown in Figure 8a suggests that the I−V curve for the sample obtained at 100 °C is higher than those for both the samples obtained at 90 and 120 °C. The nanosheet arrays obtained at 90 °C show a photocurrent density of 1.27 mA/cm2 at 1.6 V vs RHE, which is somewhat lower than that for the nanosheet arrays obtained at 100 °C (1.79 mA/cm2 at 1.6 V vs RHE). The irregular G



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α-Fe2O3 nanosheet arrays may represent a promising photoanode for PEC water oxidation. It may be reasonably expected that the PEC performance of the α-Fe2O3 nanosheet arrays could be further improved through optimizing the synthesis conditions combined with various chemical modification methods, such as improving the doping technique, surface modification with catalysts or passivation layers, heterojunction construction, and introduction of underlayers or scaffolds.

CONCLUSIONS Novel ferrihydrite branched nanosheet arrays vertically aligned on FTO substrate were fabricated through a facile one-step solvothermal method under mild conditions (5 h at 100 °C). The hierarchical branched ferrihydrite nanosheet arrays consisted of tiny branches up to 40 nm in length grown almost vertically on stem nanosheets ∼10 nm in thickness. Moreover, the variation of the morphology of the ferrihydrite nanostructures from bare nanosheet arrays through branched nanosheet arrays to dense branched structures without stem nanosheets can be readily achieved through the regulation of the reaction time and temperature. It was proposed that there may exist competitive adsorption on the growing ferrihydrite crystals between F− ions and acac group, and the presence of F− ions favored the preferential growth of the stem nanosheets, whereas the gradual release of acetylacetonate group promoted the subsequent growth of the side branches. The obtained ferrihydrite branched nanosheet arrays were transformed into α-Fe2O3 nanosheet arrays with small surface protrusions upon annealing at 550 °C. After a simple postgrowth Ti-doping process, the Ti-doped α-Fe2O3 nanosheet arrays showed a good PEC performance for water splitting with a photocurrent density of 1.79 mA/cm2 at 1.6 V vs RHE. The EIS and Mott−Schottky analysis demonstrated an increase in the interface charge transfer rate and donor density of the nanosheet array photoanode after Ti-doping. Furthermore, investigation on the photocurrent curves and EIS Nyquist plots of the α-Fe2O3 nanosheet arrays obtained at different temperatures revealed that the morphology of αFe2O3 photoanodes plays a key role in determining their PEC performance. The PEC performance of the unique α-Fe2O3 nanosheet arrays could be further enhanced by combination with various chemical modification methods and the novel αFe2O3 nanosheet arrays may represent a promising candidate for efficient photoanodes toward PEC water splitting. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08116. SEM images and FTIR spectra of ferrihydrite nanostructures obtained under various synthesis conditions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (Grants 21173010 and 21473004) and MOST (Grant 2013CB932601). H


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Controlled Growth of Ferrihydrite Branched Nanosheet Arrays and

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