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Potent and environmental-friendly l -cysteine @ Fe2O3 nanostructure for photoelectrochemical water splitting
Electrochimica Acta 259 (2018) 86e93
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Potent and environmental-friendly L-cysteine @ Fe2O3 nanostructure for photoelectrochemical water splitting Ping Qiu*, Hongfei Yang, Yu Song, Lianjie Yang, Lijun Lv, Xiong Zhao, Lei Ge**, Changfeng Chen Department of Materials Science and Engineering, Beijing Key Laboratory of Failure, Corrosion and Protection of Oil/Gas Facilities, China University of Petroleum (Beijing), 18 Fuxue Road, Changping, Beijing, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2017 Received in revised form 26 July 2017 Accepted 26 October 2017 Available online 26 October 2017
This work has reported a simple, environmental-friendly, low-cost and relatively high efficient method of L-cys @ Fe2O3 NTs photoanode preparation. The designed specimen was composed of hematite nanotubes obtained by electrochemical anodization on mild steel, which surface was covered by L-cysteine thin layer. With the overlayer, the water oxidation photocurrent of hematite increased approximately sixfold (1.23 V vs RHE (reversible hydrogen electrode) under 1 sun illumination). To reveal the enhanced PEC performance, multiple analyses including the sample surface morphological and chemical properties, optical and electronic properties, and photoelectrochemical properties were explored. The combined results suggested that L-cys @ Fe2O3 NTs photoanode presented more efficient hole migration pathway, lower interfacial resistance, and superior surface stability than pure nanostructured hematite. © 2017 Elsevier Ltd. All rights reserved.
Keywords: L-cysteine Fe2O3 nanotubes Mild steel Water splitting
1. Introduction The diurnal solar energy supplies all the needs to the Earth in essence. Fujishima and Honda have firstly reported in 1972 that it could supply promising and stable energy resource for PEC, which harvests the energy and store in available chemical forms via solar water splitting [1]. To achieve this, there are extensive studies focused on photoanodes synthesis with properties of nontoxicity, proper band gap, easy accessibility, low-cost and high efficiency in water splitting reaction [2e5]. Among these materials, hematite (Fe2O3) is the prime candidate for wide application, because it satisfies those numerous stringent requirements due to its suitable band gap (about 2.0 eV) and wide distribution in earth. However, it still has drawbacks, such as short hole diffusion distance, low visible light absorption coefficient and the presence of surface trapping states at the interface of semiconductor and electrolyte [6e9]. Some studies show that one-dimensional nanostructure of Fe2O3 would be a wonderful solution to shorten the charge carrier migration distance [10e12]. To reduce the surface holes trapping
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Qiu), [email protected] (L. Ge). https://doi.org/10.1016/j.electacta.2017.10.168 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
states, researchers have deposited various overlayers on the hematite thin film, e.g. Al2O3 [13] or p-type hematite [14]. Furthermore, there are also reports on supplying organic sacrificial electron donors EDTA2 for TiO2 photoanode on PEC water splitting study [15]. All these methods can facilitate holes participate in oxygen evolution reaction (OER) and enhance the solar energy conversion efficiencies. In considering this, we applied a simple and reproducible method to improve hole migration pathway, reduce interfacial resistance between photoanode and electrolyte, and enhance surface stability of Fe2O3. This study has prepared Fe2O3 nanotubes (NTs) on mild steel, which is one of the most low-cost steels in industrial applications, by electrochemical anodization. And then the surface was treated by L-cysteine adsorption. L-cysteine is an important amino acid which is biodegradable and easy to obtain [16]. The application of this molecule has attracted many attentions, due to its distinctive interfacial behavior at the interface of metal and solution, such as electrode modifier [17]. Based on the designed photoanode, we found that this structure presented enhanced OER performance. This method may supply one potential approach toward solar energy conversion on PEC water splitting for industrial use.
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2. Experimental 2.1. Preparation of Fe2O3 NTs The Fe2O3 NTs were prepared on mild steel by potentiostatic anodization based on a previous report [18]. The chemical composition (wt. %) of the steel used in this work is as follows: C (0.14), Si (0.30), Mn (0.5), P (0.05), S (0.04). The specimen (10 mm 15 mm 0.4 mm) was abraded with silicon carbide paper down to 2000 mesh and then polished with diamond paste down to 1 mm. Finally, it was ultrasonically in ethanol and acetone, and flushed with N2 gas. Each specimen has a surface area of 1 cm2 to contact the solution. The potentiostatic anodization was performed with a DC current power station at the potential of 50 V for 450 s. A two-electrode system was applied, which included the steel specimen as anode and platinum foil as counter electrode. The electrolyte medium consisted of ethylene glycol þ1.2 M ammonium fluoride (NH4F) þ 3.5 vol % Millipore water (MQ water, 18 MU), which was aerated by compressing air for 30 min before each test. The anodization bath temperature was controlled at 30 C by water bath. Subsequently, the specimen was ultrasound cleaned with MQ water for 10 s, and then purged with nitrogen gas. In order to improve the crystallinity of the sample, it was kept in the muffle furnace at 450 C for 1 h in air. After natural cooling to room temperature, the specimen was stored in a dry and dark cabinet. 2.2. Preparation of L-cys @ Fe2O3 NTs 100 mL L-cysteine (8.3 mM) solution was evenly distributed onto the prepared specimen surface by a transfer pipette. And then the specimen was heated in a temperature-controlled electric oven at 100 C for 10 min to evaporate water. After natural cooling to room temperature, the L-cys @ Fe2O3 NTs photoanode was ready for use. 2.3. Photoelectrochemical and electrochemical measurements Zahner IM6eX station was applied to perform current densitypotential (J-V) scan, electrochemical impedance spectroscopy (EIS), chronoamperometry and chronopotentiometry measurements. A three-electrode system was used with a saturated Ag/AgCl electrode as the reference electrode, platinum wire as the counter electrode, and the prepared photoanodes as the working electrode. To meet the experimental requirements, the measurements were performed in quartz PEC cell filled with electrolyte of 1 M NaOH (pH ¼ 13.6, 25 C, 150 mL). A 150 W xenon lamp equipped with filter (AM 1.5G) was used as the light source. The power density was 100 mW/cm2 and determined by a radiometer. J-V scan was performed at a scan rate of 50 mV/s in the range of
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0.7e1.5 V vs RHE (the measured potentials were converted to the RHE by Nernst formula ERHE ¼ EAg/AgCl þ 0.059 pH þ E0 Ag/AgCl). The EIS spectra were recorded at DC potential of 1.23 V vs RHE in 1 M NaOH solution with a sinusoidal perturbation of 10 mV and frequency range from 105 to 102 Hz under illumination. Then the recorded impedance data was analyzed and fitted with Zview software. Chronoamperometry (current vs time) test was carried out at 1.23 V vs RHE in 1 M NaOH media under illumination. Chronopotentiometry (potential vs time) curves were recorded at a constant current density of 10 mA/cm2 in 1 M NaOH media under dark condition. 2.4. Surface and chemical characterization Scanning electron microscope (SEM) measurement was conducted by FEI Quanta 200 F at the acceleration voltage of 5 kV to study the morphology of the annealed specimen. The surface chemical composition of the as-anodized and annealed specimen was determined by confocal Raman microspectroscopy (CRM) using HORIBAHR-800 system equipped with a laser source of wavelength 514 nm. And the scanning range was from 100 cm1 to 1000 cm1. The spectral resolution was 1e2 cm1. The chemical bonding state of L-cysteine with the annealed nanostructured surface was characterized with Fourier transform infrared spectroscopy (FT-IR) of the Bruker HYPERION 3000 microscope. The FT-IR spectra were recorded in the range from 600 cm1 to 3000 cm1 with an interval of 0.5 cm1 and resolution of 4 cm1. X-ray photoelectron spectroscopy (XPS) analysis was performed by a PHI Quantera SXM with a monochromatic Al Ka X-ray source. The binding energies obtained in the XPS spectral analysis were corrected for specimen by referencing C 1s to 284.6 eV. Contact angles of the photoanode surfaces were determined by sessile drop method using an optical contact angle meter SL200KB. The steady-state average value of left and right contact angles was regarded as the contact angle value. Optical absorbance was determined by a double beam Hitachi-330 UVeVisible spectrophotometer in the range of 250e800 nm. 2.5. Quantum chemical study Quantum chemical calculations were conducted using Materials StudioDMol3 software (Accelrys Inc.). The double-numerical plus polarization (DNP) basis sets were used. The electronic density distribution of the highest occupied molecular orbital (HOMO) of geometrically optimized L-cysteine molecular were presented. 3. Results and discussion The Fe2O3 NTs have been prepared by electrochemical
Fig. 1. Schematic shows the formation of nanotube structure on mild steel by electrochemical anodization at 50 V for 450 s in ethylene glycol solution with 1.2 M NH4F and 3.5 vol % H2O.
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anodization method in weak acidic organic electrolyte. It has been reported that the solvent of ethylene glycol can promote a slowly uniform passivation on the steel substrate [18e21], which is the initial step for the nanostructures formation, as indicated in Fig. 1 step I. During the formation of the passive film, the fluoride anion in the electrolyte can dispersed within the film, which induces the defects and strains (step II). This is similar to the pitting process caused by chloride ion [22], it becomes active dissolution center of the film resulting in the self-organized Fe2O3 NTs (step III). Fig. 2(aec) shows the representative SEM images of the nanotube formed on mild steel after anodic oxidation process. In the top view image, the ordered nanotube presents an inside diameter of about 35 nm. By observing the corresponding cross-sectional image, the average length of the nanotube is about 740 nm. The chemical composition of the as-anodized and annealed nanotube has been characterized by CRM, as shown in Fig. 2 (d). There are
mainly two peaks appeared for the as-anodized specimen in the CRM spectrum, which could be assigned as g-Fe2O3 (718 cm1) and Fe3O4 (656 cm1). The identified compounds are a-Fe2O3 (220 cm1, 292 cm1, 405 cm1, 499 cm1 and 610 cm1), g-Fe2O3 (667 cm1 and 718 cm1) and Fe3O4 (656 cm1) on the spectrum of annealed nanotube specimen [23,24]. Furthermore, the corresponding bands are found to increase in intensity and decrease in linewidth comparing with the as-anodized sample. This suggests that annealing treatment could improve the nanostructure crystallinity. The OER activity of the as prepared Fe2O3 NTs and L-cys @ Fe2O3 NTs electrodes in alkaline solution under 1 sun illumination is evaluated by a three-electrode electrochemical cell. The J-V curves are shown in Fig. 3 (a). The photocurrent of L-cys @ Fe2O3 NTs electrode (0.522 mA/cm2 at 1.23 V vs RHE) increases about sixfold in comparison with Fe2O3 NTs electrode (0.082 mA/cm2 at 1.23 V vs
Fig. 2. Top (a, b) and cross-sectional (c) view of SEM images of annealed hematite nanotubes prepared at 50 V for 450 s in ethylene glycol solution with 1.2 M NH4F and 3.5 vol % H2O on mild steel; Confocal Raman microspectroscopy (d) of the surface composition compared between the as-anodized and annealed sample.
Fig. 3. PEC current density-potential curves (a) of L-cys @ Fe2O3 NTs and Fe2O3 NTs under 1 sun illumination and in the dark condition; Photocurrent density curves (b) of L-cys @ Fe2O3 NTs and Fe2O3 NTs recorded at 1.23 V vs RHE under 1 sun illumination for 5000 s.
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RHE). And the corresponding current onset potential shifts to the cathodic direction by 0.06 V. So the designed L-cys @ Fe2O3 NTs presents an efficient catalytic behavior for solar water oxidation. Furthermore, we can observe that the anodic depolarization process is enhanced for L-cys on Fe2O3 electrode than Fe2O3 NTs electrode. This implies that the adsorption of L-cys on Fe2O3 electrode surface may facilitate hole transfer at the interface of electrode and the electrolyte. The photocurrent density of L-cys @ Fe2O3 NTs and Fe2O3 NTs were studied at 1.23 V vs RHE in 1 M NaOH media under illumination over a period of 5000 s. The results are displayed in Fig. 3 (b). The current density of L-cys @ Fe2O3 NTs does not show obvious decay in the tested period. However, the value changes from 0.091 to 0.070 mA/cm2 for Fe2O3 NTs sample. The results demonstrate
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that L-cys @ Fe2O3 NTs is more stable than Fe2O3 NTs in alkaline media. To further understand the charge transfer behavior at the interface, the EIS analysis was performed at 1.23 V vs RHE in 1 M NaOH media under one sun illumination. Fig. 4 (a) is the typical Nyquist plots for Fe2O3 NTs and L-cys @ Fe2O3 NTs. Due to the electrical conductive property of Fe2O3 NTs and L-cys @ Fe2O3 NTs, the space charge layer capacitance of the sample should be considered. An equivalent electrical circuit consisting of 2 RC elements is used for EIS data fitting, as shown in Fig. 4 (b). In this model, RS caused from the resistance of FTO, electrolyte, and electrical cables, RCT represents the charge transfer resistance at the interface of the photoanode and electrolyte, CPEH represents the constant phase element of Helmholtz layer, RSC and CPESC describe
Fig. 4. EIS plots (a) recorded on L-cys @ Fe2O3 NTs and Fe2O3 NTs at 1.23 V vs RHE under 1 sun illumination; The equivalent circuit model (b) used to fit the EIS plots.
Table 1 Parameters obtained by simulating the EIS data.
Fe2O3 NTs L-cys @ Fe2O3 NTs
RS (U)
CPEH Y0 (U1$sn)
n
2.69 1.70
2.98 103 5.01 103
0.51 0.65
RCT (U)
CPESC Y0 (U1$sn)
n
102.10 25.03
5.72 104 8.72 104
0.85 0.84
RSC (U) 12084 2095
Fig. 5. The scheme of the molecular structure of L-cysteine powder (a, left) and ionization of L-cysteine in aqueous alkaline media (a, right); the highest occupied molecular orbital density (HOMO) for L-cysteine (b); FT-IR spectra of pure L-cysteine powder (c, green top line) and L-cys @ Fe2O3 NTs specimen after the PEC test in 1 M NaOH media under 1 sun illumination (c, red bottom line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the resistance and constant phase element of the photoanode space charge layer [25e27]. CPE is the constant phase element which denotes the deviation from the ideal capacitance owing to the sample surface inhomogeneity. The corresponding fitted values were listed in Table 1. The RCT of L-cys @ Fe2O3 NTs demonstrates a dramatic drop comparing with Fe2O3 NTs. This could be attributed to the efficient charge transfer. The above results are consistent with those obtained from the J-V curves. Since flux of holes injection is related to the space charge layer, this process is associated with the RSC, which is much smaller for Lcys @ Fe2O3 NTs than that of Fe2O3 NTs. So the conductivity of L-cys @ Fe2O3 NTs is less than Fe2O3 NTs. Moreover, the value of CPESC for
L-cys @ Fe2O3 NTs is higher than that of Fe2O3 NTs. This may imply that the electron Fermi level of the hematite could approach more close to its flat-band potential for L-cys @ Fe2O3 NTs, leading to a decreased space charge layer thickness consequently a higher capacitance. Hager and Lazarescu's studies have denoted that the negatively charged species become prevail in alkaline medium for L-cysteine, as displayed in Fig. 5 (a) [28e30]. In general, a coordinate covalent bond is formed by transferring electrons from the nucleophile center of the molecule to the substrate after deposition [31,32]. In order to get insight on the mechanism of the observed enhanced water splitting behavior, the chemical bonding state of L-cysteine
Fig. 6. Survey and high-resolution XPS spectra of the Fe2O3 NTs and L-cys @ Fe2O3 NTs after the PEC test in 1 M NaOH media under 1 sun illumination and the pure L-cysteine powder, respectively.
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on the hematite surface was studied by FT-IR and XPS. Fig. 5 (c) presents the typical IR spectrum recorded on L-cysteine @ Fe2O3. For comparison, the IR spectrum of L-cysteine powders is also shown. According to earlier study on peak assignments of Lcysteine by IR [33], the peaks in the regions of 1580 cm1 and 1388 cm1 correspond to the asymmetric and symmetric carboxylate group (COO) stretching vibration. The presence of thiol group (SeH) can be proved by the peaks at 2550 cm1 and 940 cm1representing to its stretching vibration. The peaks at 1346 cm1 and 1140 cm1 can be assigned to the amino group (eNH2). Comparing with the spectrum of pure L-cysteine, the SeH vibration band is not detected for the specimen of L-cys @ Fe2O3 NTs. The absence of this band demonstrates that L-cysteine is bonded to the substrate through sulfur atom [34]. It has been reported that the relative intensities of the carboxylate bands can be used to infer the orientation of the carboxylate species according to the IR surface selection rule [35]. In this study, the relative intensities of asymmetric carboxylate stretching band is lower than the symmetric one, which presents obviously different relationship comparing to the L-cysteine powders. The corresponding change in the spectrum may indicate the coordination of the carboxylate group to the substrate. So FT-IR spectra analysis suggests that Lcysteine could bond to the Fe2O3 surface through thiol and carboxylate group. To further clarify this molecular bonding mechanism, quantum calculations were adopted. The electron density distribution of the HOMO (Fig. 5 (b)) can depict the nucleophile center of L-cysteine molecule. It could be observed that the electronic densities focus on the sulfur and oxygen atoms. So the nucleophile centers of the molecule are sulfur and oxygen atoms, which could keep bonding to the positively charged substrate under anodic polarization and illumination condition. Furthermore, the same sample was also studied by XPS shown in Fig. 6. From the survey spectra (Fig. 6 (a)), the existence of C, Fe, O, S and N are confirmed. The high-resolution C 1s XPS spectra (Fig. 6 (b)) of Fe2O3 NTs can be deconvoluted to two kinds of carbon species based on Gaussian-Lorentzian fitting: the adventitious carbon adsorbs on the surface of the sample (284.6 eV) and C]O bond (288.2 eV) is detected attributed to CO2 adsorption [36,37]. But for the high-resolution C 1s XPS spectra of L-cys @ Fe2O3 NTs and pure L-cysteine powder specimen, there are three carbon species, CeS (285.2 eV), CeN (286.1 eV) and COOH/COO (288.0 eV) [38,39]. And in the high resolution spectra of Fe 2p (Fig. 6 (c)), FeO (709.8 eV), Fe2O3 (710.6 eV) and FeOOH (711.5 eV) can be recognized for both samples [40,41]. FeS (709.2 eV) can also be detected on the surface of L-cys @ Fe2O3 NTs [39]. The high resolution spectra of O1s (Fig. 6 (d)) can be deconvoluted into three types, O2 (528.5 eV), OH (530.1 eV, represents the surface hydroxyl groups) and eCOOH (530.9 eV, carboxylic group), respectively [39,42e44]. As can be seen, L-cys @ Fe2O3 NTs surface presents a higher percentage of OH than that of Fe2O3 NTs. In Fig. 6 (e), the detected of S 2p peak implies the presence of sulfides on surface of L-cys @ Fe2O3 NTs: -S2-(161.8 eV), S2- 2 (163.5 eV) and S6þ (186.5 eV) [45,46]. The presented S6þ suggests a strong chemical bonding is formed on the sample surface. The high resolution of N1s spectra (shown in Fig. 6
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(f) suggests that there is no distinct change for the group of NH2 after PEC measurements. So it can conclude that the adsorption of Lcysteine on Fe2O3 NTs is essentially stable even after PEC measurements. And it could highlight the surface state modification due to L-cysteine adsorption on Fe2O3 NTs surface. This observation is agreed with Somorjai etc., reported that the chemisorption of small molecules on the substrate can induce the surface restructuring [47]. In current experimental condition, the photoanode surface wettability has an important influence on PEC performance. The contact angle measurement is a method to characterize the surface wettability to some extent [48]. The measured contact angle images are shown in Fig. 7, which have been collected on L-cys @ Fe2O3 NTs and Fe2O3 NTs. It can be observed that the smaller average contact angle (7 ) appears on the surface of L-cys @ Fe2O3 NTs in comparison with Fe2O3 NTs sample (32 ), demonstrating its better wettability and much easier access of the aqueous electrolyte to the active sites [49]. It is well accepted that optical absorption of photoanode significantly influences their activity. The optical properties of L-cys @ Fe2O3 NTs and Fe2O3 NTs were estimated by using UVeVisible absorption spectroscopy. Fig. 8 shows the absorbance variations as a function of wavelength between 200 and 800 nm. It can be seen that L-cys @ Fe2O3 NTs has higher optical absorption than Fe2O3 NTs, both of which cover the UV and visible region. Based on the reported band assignments, the typical absorption bands appeared in region 250e400 nm are attributed to the ligand-to-meal charge transfer transitions with combined contributions from the Fe3þ ligand field transitions 6A1(6S) to 4T1(4P) at 290e310 nm, 6A1(6S) to 4 E (4D) and 6A1(6S) to 4T1 (4D) at 360e380 nm. The absorption bands in the region of 400e600 nm are due to the double excitation processes 6A1(6S)þ 6A1(6S) to 4T1 (4G)þ 4T1 (4G) at 480e550 nm, which could partly overlap with the ligand filed transition 6A1(6S) to 4E (4D) and 6A1(6S) to 4A1 (4G) at 430 nm [50,51]. Furthermore,
Fig. 8. UVeVisible absorption spectra of Fe2O3 NTs and L-cys @ Fe2O3 NTs specimen.
Fig. 7. Contact angle images of Fe2O3 NTs (a) and L-cys @ Fe2O3 (b) NTs.
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Fig. 9. Chronopotentiometry curves of L-cys @ Fe2O3 NTs and Fe2O3 NTs recorded at 10 mA/cm2 in 1 M NaOH media under dark; (b) Schematic model of the PEC water splitting occurred on L-cys @Fe2O3 NTs surface.
the band gap values have been determined from the spectra extrapolations: L-cys @ Fe2O3 NTs (1.99 eV) and Fe2O3 NTs (1.94 eV), which is consistent with the literature reports [6,51,52]. There is no significant band gap shift by L-cysteine deposition on Fe2O3 NTs surface. The absorption spectra results are consistent with improved photocurrent performance observed on L-cys @ Fe2O3 NTs sample. Chronopotentiometry measurement was performed at a constant current density of 10 mA/cm2 on L-cys @ Fe2O3 NTs and Fe2O3 NTs in 1 M NaOH solution under dark condition. The recorded curves are shown in Fig. 9 (a). Apparently, there is no obvious potential degradation observed on the L-cys @ Fe2O3 NTs over 5000 s continuous monitoring. But the potential of Fe2O3 NTs sample shifts about 74 mV to lower potential. Earlier studies have reported that water oxidation on hematite is mediated by the surface formation of iron complex [9,27,53,54]. The iron complex is unstable under high potential polarization. Hence, it's can be considered that Lcysteine acts as curing agent on the hematite surface. Hematite is one of the most prime candidates for photoanodes in PEC water splitting. The low visible light absorption coefficient and short hole migration length are the main drawbacks and limit its PEC performance. The main contribution of this work is to adopt a simple method on hematite photoanode fabrication, which helps to improve its main drawbacks. The fabricated Fe2O3 nanotubes are in well-ordered array. This one-dimensional nanostructure can facilitate hole migration. However, this is not sufficient for the separation of the pair. So the corresponding PEC activity is still limited. In considering this, we deposited a small quantity of Lcysteine on the nanostructured hematite surface. This amino molecule could chemical bonding on the substrate through thiol and carboxylate group. By adsorption, it can not only help hematite to resist corrosion in alkaline solution but also act as conductive linker. Under anodic polarization and illumination condition, the injection of hole could be effectively transferred to the electrolyte to promote water oxidation at the hematite surface. A model is proposed to illustrate the corresponding mechanism, as shown in Fig. 9 (b).
4. Conclusions In this study, we have prepared well arrayed hematite nanotube by electrochemical anodization on mild steel. When applying Lcysteine on the corresponding nanostructure, a potent behavior on PEC water splitting is observed. The enhanced performance is attributed to the modified surface states by chemical bonding of the nucleophile center including carboxylate and thiol group to the substrate. Then L-cysteine acts as a conductive linker and surface
curing agent to facilitate the flux of hole transfer. The highlight of this study is to supply a simple and effective method of L-cys @ Fe2O3 NTs photoanode preparation for PEC water splitting and attempt to explain the mechanism. Acknowledgments This work was supported by the National Natural Science Foundation of China [Grant No. 51301199] and the funding of China University of Petroleum (Beijing) [Grant No. 2462015YQ0602]. We are also grateful to the reviewers for the helpful comments. We also thanks Dr. Xueyuan Zhang (Gamry Instruments) for valuable suggestions. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37e38. [2] M.T. Mayer, C. Du, D. Wang, Hematite/Si nanowire dual-absorber system for photoelectrochemical water splitting at low applied potentials, J. Am. Chem. Soc. 134 (2012) 12406e12409. [3] G. Rahman, O.-S. Joo, Photoelectrochemical water splitting at nanostructured a-Fe2O3 electrodes, Int. J. Hydrogen Energy 37 (2012) 13989e13997. [4] S. Begonja, L.A.G. Rodenas, E.B. Borghi, P.J. Morando, Adsorption of cysteine on TiO2 at different pH values: surface complexes characterization by FTIR-ATR and Langmuir isotherms analysis, Colloids Surf. A Physicochem. Eng. Asp. 403 (2012) 114e120. pez, D. Monllor-Satoca, J.D. Prades, M.D. Hern [5] C. F abrega, S. Murcia-Lo andezAlonso, G. Penelas, J.R. Morante, T. Andreu, Efficient WO3 photoanodes fabricated by pulsed laser deposition for photoelectrochemical water splitting with high faradaic efficiency, Appl. Catal. B Environ. 189 (2016) 133e140. €tzel, Solar water splitting: progress using he[6] K. Sivula, F. Le Formal, M. Gra matite (a-Fe2O3) photoelectrodes, ChemSusChem 4 (2011) 432e449. [7] J.H. Kennedy, K.W. Frese, Photooxidation of water at a - Fe2 O 3 electrodes, J. Electrochem. Soc. 125 (1978) 709e714. [8] K.X. Wang, Z. Yu, V. Liu, M.L. Brongersma, T.F. Jaramillo, S. Fan, Nearly total solar absorption in ultrathin nanostructured iron oxide for efficient photoelectrochemical water splitting, ACS Photonics 1 (2014) 235e240. [9] B. Klahr, S. Gimenez, F. Fabregat-Santiago, T. Hamann, J. Bisquert, Water oxidation at hematite photoelectrodes: the role of surface states, J. Am. Chem. Soc. 134 (2012) 4294e4302. [10] H.K. Mulmudi, N. Mathews, X.C. Dou, L.F. Xi, S.S. Pramana, Y.M. Lam, S.G. Mhaisalkar, Controlled growth of hematite (a-Fe2O3) nanorod array on fluorine doped tin oxide: synthesis and photoelectrochemical properties, Electrochem. Commun. 13 (2011) 951e954. [11] A. Annamalai, P.S. Shinde, T.H. Jeon, H.H. Lee, H.G. Kim, W. Choi, J.S. Jang, Fabrication of superior a-Fe2O3 nanorod photoanodes through ex-situ Sndoping for solar water splitting, Sol. Energy Mater. Sol. Cells 144 (2016) 247e255. [12] Z. Zhang, M.F. Hossain, T. Takahashi, Self-assembled hematite (a-Fe2O3) nanotube arrays for photoelectrocatalytic degradation of azo dye under simulated solar light irradiation, Appl. Catal. B Environ. 95 (2010) 423e429. [13] F. Le Formal, N. Tetreault, M. Cornuz, T. Moehl, M. Gratzel, K. Sivula, Passivating surface states on water splitting hematite photoanodes with alumina overlayers, Chem. Sci. 2 (2011) 737e743. [14] Y. Lin, Y. Xu, M.T. Mayer, Z.I. Simpson, G. McMahon, S. Zhou, D. Wang, Growth of p-type hematite by atomic layer deposition and its utilization for improved
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Potent and environmental-friendly L-cysteine @ Fe2O3 nanostructure for photoelectrochemical water splitting Ping Qiu*, Hongfei Yang, Yu Song, Lianjie Yang, Lijun Lv, Xiong Zhao, Lei Ge**, Changfeng Chen Department of Materials Science and Engineering, Beijing Key Laboratory of Failure, Corrosion and Protection of Oil/Gas Facilities, China University of Petroleum (Beijing), 18 Fuxue Road, Changping, Beijing, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2017 Received in revised form 26 July 2017 Accepted 26 October 2017 Available online 26 October 2017
This work has reported a simple, environmental-friendly, low-cost and relatively high efficient method of L-cys @ Fe2O3 NTs photoanode preparation. The designed specimen was composed of hematite nanotubes obtained by electrochemical anodization on mild steel, which surface was covered by L-cysteine thin layer. With the overlayer, the water oxidation photocurrent of hematite increased approximately sixfold (1.23 V vs RHE (reversible hydrogen electrode) under 1 sun illumination). To reveal the enhanced PEC performance, multiple analyses including the sample surface morphological and chemical properties, optical and electronic properties, and photoelectrochemical properties were explored. The combined results suggested that L-cys @ Fe2O3 NTs photoanode presented more efficient hole migration pathway, lower interfacial resistance, and superior surface stability than pure nanostructured hematite. © 2017 Elsevier Ltd. All rights reserved.
Keywords: L-cysteine Fe2O3 nanotubes Mild steel Water splitting
1. Introduction The diurnal solar energy supplies all the needs to the Earth in essence. Fujishima and Honda have firstly reported in 1972 that it could supply promising and stable energy resource for PEC, which harvests the energy and store in available chemical forms via solar water splitting [1]. To achieve this, there are extensive studies focused on photoanodes synthesis with properties of nontoxicity, proper band gap, easy accessibility, low-cost and high efficiency in water splitting reaction [2e5]. Among these materials, hematite (Fe2O3) is the prime candidate for wide application, because it satisfies those numerous stringent requirements due to its suitable band gap (about 2.0 eV) and wide distribution in earth. However, it still has drawbacks, such as short hole diffusion distance, low visible light absorption coefficient and the presence of surface trapping states at the interface of semiconductor and electrolyte [6e9]. Some studies show that one-dimensional nanostructure of Fe2O3 would be a wonderful solution to shorten the charge carrier migration distance [10e12]. To reduce the surface holes trapping
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Qiu), [email protected] (L. Ge). https://doi.org/10.1016/j.electacta.2017.10.168 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
states, researchers have deposited various overlayers on the hematite thin film, e.g. Al2O3 [13] or p-type hematite [14]. Furthermore, there are also reports on supplying organic sacrificial electron donors EDTA2 for TiO2 photoanode on PEC water splitting study [15]. All these methods can facilitate holes participate in oxygen evolution reaction (OER) and enhance the solar energy conversion efficiencies. In considering this, we applied a simple and reproducible method to improve hole migration pathway, reduce interfacial resistance between photoanode and electrolyte, and enhance surface stability of Fe2O3. This study has prepared Fe2O3 nanotubes (NTs) on mild steel, which is one of the most low-cost steels in industrial applications, by electrochemical anodization. And then the surface was treated by L-cysteine adsorption. L-cysteine is an important amino acid which is biodegradable and easy to obtain [16]. The application of this molecule has attracted many attentions, due to its distinctive interfacial behavior at the interface of metal and solution, such as electrode modifier [17]. Based on the designed photoanode, we found that this structure presented enhanced OER performance. This method may supply one potential approach toward solar energy conversion on PEC water splitting for industrial use.
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2. Experimental 2.1. Preparation of Fe2O3 NTs The Fe2O3 NTs were prepared on mild steel by potentiostatic anodization based on a previous report [18]. The chemical composition (wt. %) of the steel used in this work is as follows: C (0.14), Si (0.30), Mn (0.5), P (0.05), S (0.04). The specimen (10 mm 15 mm 0.4 mm) was abraded with silicon carbide paper down to 2000 mesh and then polished with diamond paste down to 1 mm. Finally, it was ultrasonically in ethanol and acetone, and flushed with N2 gas. Each specimen has a surface area of 1 cm2 to contact the solution. The potentiostatic anodization was performed with a DC current power station at the potential of 50 V for 450 s. A two-electrode system was applied, which included the steel specimen as anode and platinum foil as counter electrode. The electrolyte medium consisted of ethylene glycol þ1.2 M ammonium fluoride (NH4F) þ 3.5 vol % Millipore water (MQ water, 18 MU), which was aerated by compressing air for 30 min before each test. The anodization bath temperature was controlled at 30 C by water bath. Subsequently, the specimen was ultrasound cleaned with MQ water for 10 s, and then purged with nitrogen gas. In order to improve the crystallinity of the sample, it was kept in the muffle furnace at 450 C for 1 h in air. After natural cooling to room temperature, the specimen was stored in a dry and dark cabinet. 2.2. Preparation of L-cys @ Fe2O3 NTs 100 mL L-cysteine (8.3 mM) solution was evenly distributed onto the prepared specimen surface by a transfer pipette. And then the specimen was heated in a temperature-controlled electric oven at 100 C for 10 min to evaporate water. After natural cooling to room temperature, the L-cys @ Fe2O3 NTs photoanode was ready for use. 2.3. Photoelectrochemical and electrochemical measurements Zahner IM6eX station was applied to perform current densitypotential (J-V) scan, electrochemical impedance spectroscopy (EIS), chronoamperometry and chronopotentiometry measurements. A three-electrode system was used with a saturated Ag/AgCl electrode as the reference electrode, platinum wire as the counter electrode, and the prepared photoanodes as the working electrode. To meet the experimental requirements, the measurements were performed in quartz PEC cell filled with electrolyte of 1 M NaOH (pH ¼ 13.6, 25 C, 150 mL). A 150 W xenon lamp equipped with filter (AM 1.5G) was used as the light source. The power density was 100 mW/cm2 and determined by a radiometer. J-V scan was performed at a scan rate of 50 mV/s in the range of
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0.7e1.5 V vs RHE (the measured potentials were converted to the RHE by Nernst formula ERHE ¼ EAg/AgCl þ 0.059 pH þ E0 Ag/AgCl). The EIS spectra were recorded at DC potential of 1.23 V vs RHE in 1 M NaOH solution with a sinusoidal perturbation of 10 mV and frequency range from 105 to 102 Hz under illumination. Then the recorded impedance data was analyzed and fitted with Zview software. Chronoamperometry (current vs time) test was carried out at 1.23 V vs RHE in 1 M NaOH media under illumination. Chronopotentiometry (potential vs time) curves were recorded at a constant current density of 10 mA/cm2 in 1 M NaOH media under dark condition. 2.4. Surface and chemical characterization Scanning electron microscope (SEM) measurement was conducted by FEI Quanta 200 F at the acceleration voltage of 5 kV to study the morphology of the annealed specimen. The surface chemical composition of the as-anodized and annealed specimen was determined by confocal Raman microspectroscopy (CRM) using HORIBAHR-800 system equipped with a laser source of wavelength 514 nm. And the scanning range was from 100 cm1 to 1000 cm1. The spectral resolution was 1e2 cm1. The chemical bonding state of L-cysteine with the annealed nanostructured surface was characterized with Fourier transform infrared spectroscopy (FT-IR) of the Bruker HYPERION 3000 microscope. The FT-IR spectra were recorded in the range from 600 cm1 to 3000 cm1 with an interval of 0.5 cm1 and resolution of 4 cm1. X-ray photoelectron spectroscopy (XPS) analysis was performed by a PHI Quantera SXM with a monochromatic Al Ka X-ray source. The binding energies obtained in the XPS spectral analysis were corrected for specimen by referencing C 1s to 284.6 eV. Contact angles of the photoanode surfaces were determined by sessile drop method using an optical contact angle meter SL200KB. The steady-state average value of left and right contact angles was regarded as the contact angle value. Optical absorbance was determined by a double beam Hitachi-330 UVeVisible spectrophotometer in the range of 250e800 nm. 2.5. Quantum chemical study Quantum chemical calculations were conducted using Materials StudioDMol3 software (Accelrys Inc.). The double-numerical plus polarization (DNP) basis sets were used. The electronic density distribution of the highest occupied molecular orbital (HOMO) of geometrically optimized L-cysteine molecular were presented. 3. Results and discussion The Fe2O3 NTs have been prepared by electrochemical
Fig. 1. Schematic shows the formation of nanotube structure on mild steel by electrochemical anodization at 50 V for 450 s in ethylene glycol solution with 1.2 M NH4F and 3.5 vol % H2O.
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anodization method in weak acidic organic electrolyte. It has been reported that the solvent of ethylene glycol can promote a slowly uniform passivation on the steel substrate [18e21], which is the initial step for the nanostructures formation, as indicated in Fig. 1 step I. During the formation of the passive film, the fluoride anion in the electrolyte can dispersed within the film, which induces the defects and strains (step II). This is similar to the pitting process caused by chloride ion [22], it becomes active dissolution center of the film resulting in the self-organized Fe2O3 NTs (step III). Fig. 2(aec) shows the representative SEM images of the nanotube formed on mild steel after anodic oxidation process. In the top view image, the ordered nanotube presents an inside diameter of about 35 nm. By observing the corresponding cross-sectional image, the average length of the nanotube is about 740 nm. The chemical composition of the as-anodized and annealed nanotube has been characterized by CRM, as shown in Fig. 2 (d). There are
mainly two peaks appeared for the as-anodized specimen in the CRM spectrum, which could be assigned as g-Fe2O3 (718 cm1) and Fe3O4 (656 cm1). The identified compounds are a-Fe2O3 (220 cm1, 292 cm1, 405 cm1, 499 cm1 and 610 cm1), g-Fe2O3 (667 cm1 and 718 cm1) and Fe3O4 (656 cm1) on the spectrum of annealed nanotube specimen [23,24]. Furthermore, the corresponding bands are found to increase in intensity and decrease in linewidth comparing with the as-anodized sample. This suggests that annealing treatment could improve the nanostructure crystallinity. The OER activity of the as prepared Fe2O3 NTs and L-cys @ Fe2O3 NTs electrodes in alkaline solution under 1 sun illumination is evaluated by a three-electrode electrochemical cell. The J-V curves are shown in Fig. 3 (a). The photocurrent of L-cys @ Fe2O3 NTs electrode (0.522 mA/cm2 at 1.23 V vs RHE) increases about sixfold in comparison with Fe2O3 NTs electrode (0.082 mA/cm2 at 1.23 V vs
Fig. 2. Top (a, b) and cross-sectional (c) view of SEM images of annealed hematite nanotubes prepared at 50 V for 450 s in ethylene glycol solution with 1.2 M NH4F and 3.5 vol % H2O on mild steel; Confocal Raman microspectroscopy (d) of the surface composition compared between the as-anodized and annealed sample.
Fig. 3. PEC current density-potential curves (a) of L-cys @ Fe2O3 NTs and Fe2O3 NTs under 1 sun illumination and in the dark condition; Photocurrent density curves (b) of L-cys @ Fe2O3 NTs and Fe2O3 NTs recorded at 1.23 V vs RHE under 1 sun illumination for 5000 s.
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RHE). And the corresponding current onset potential shifts to the cathodic direction by 0.06 V. So the designed L-cys @ Fe2O3 NTs presents an efficient catalytic behavior for solar water oxidation. Furthermore, we can observe that the anodic depolarization process is enhanced for L-cys on Fe2O3 electrode than Fe2O3 NTs electrode. This implies that the adsorption of L-cys on Fe2O3 electrode surface may facilitate hole transfer at the interface of electrode and the electrolyte. The photocurrent density of L-cys @ Fe2O3 NTs and Fe2O3 NTs were studied at 1.23 V vs RHE in 1 M NaOH media under illumination over a period of 5000 s. The results are displayed in Fig. 3 (b). The current density of L-cys @ Fe2O3 NTs does not show obvious decay in the tested period. However, the value changes from 0.091 to 0.070 mA/cm2 for Fe2O3 NTs sample. The results demonstrate
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that L-cys @ Fe2O3 NTs is more stable than Fe2O3 NTs in alkaline media. To further understand the charge transfer behavior at the interface, the EIS analysis was performed at 1.23 V vs RHE in 1 M NaOH media under one sun illumination. Fig. 4 (a) is the typical Nyquist plots for Fe2O3 NTs and L-cys @ Fe2O3 NTs. Due to the electrical conductive property of Fe2O3 NTs and L-cys @ Fe2O3 NTs, the space charge layer capacitance of the sample should be considered. An equivalent electrical circuit consisting of 2 RC elements is used for EIS data fitting, as shown in Fig. 4 (b). In this model, RS caused from the resistance of FTO, electrolyte, and electrical cables, RCT represents the charge transfer resistance at the interface of the photoanode and electrolyte, CPEH represents the constant phase element of Helmholtz layer, RSC and CPESC describe
Fig. 4. EIS plots (a) recorded on L-cys @ Fe2O3 NTs and Fe2O3 NTs at 1.23 V vs RHE under 1 sun illumination; The equivalent circuit model (b) used to fit the EIS plots.
Table 1 Parameters obtained by simulating the EIS data.
Fe2O3 NTs L-cys @ Fe2O3 NTs
RS (U)
CPEH Y0 (U1$sn)
n
2.69 1.70
2.98 103 5.01 103
0.51 0.65
RCT (U)
CPESC Y0 (U1$sn)
n
102.10 25.03
5.72 104 8.72 104
0.85 0.84
RSC (U) 12084 2095
Fig. 5. The scheme of the molecular structure of L-cysteine powder (a, left) and ionization of L-cysteine in aqueous alkaline media (a, right); the highest occupied molecular orbital density (HOMO) for L-cysteine (b); FT-IR spectra of pure L-cysteine powder (c, green top line) and L-cys @ Fe2O3 NTs specimen after the PEC test in 1 M NaOH media under 1 sun illumination (c, red bottom line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the resistance and constant phase element of the photoanode space charge layer [25e27]. CPE is the constant phase element which denotes the deviation from the ideal capacitance owing to the sample surface inhomogeneity. The corresponding fitted values were listed in Table 1. The RCT of L-cys @ Fe2O3 NTs demonstrates a dramatic drop comparing with Fe2O3 NTs. This could be attributed to the efficient charge transfer. The above results are consistent with those obtained from the J-V curves. Since flux of holes injection is related to the space charge layer, this process is associated with the RSC, which is much smaller for Lcys @ Fe2O3 NTs than that of Fe2O3 NTs. So the conductivity of L-cys @ Fe2O3 NTs is less than Fe2O3 NTs. Moreover, the value of CPESC for
L-cys @ Fe2O3 NTs is higher than that of Fe2O3 NTs. This may imply that the electron Fermi level of the hematite could approach more close to its flat-band potential for L-cys @ Fe2O3 NTs, leading to a decreased space charge layer thickness consequently a higher capacitance. Hager and Lazarescu's studies have denoted that the negatively charged species become prevail in alkaline medium for L-cysteine, as displayed in Fig. 5 (a) [28e30]. In general, a coordinate covalent bond is formed by transferring electrons from the nucleophile center of the molecule to the substrate after deposition [31,32]. In order to get insight on the mechanism of the observed enhanced water splitting behavior, the chemical bonding state of L-cysteine
Fig. 6. Survey and high-resolution XPS spectra of the Fe2O3 NTs and L-cys @ Fe2O3 NTs after the PEC test in 1 M NaOH media under 1 sun illumination and the pure L-cysteine powder, respectively.
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on the hematite surface was studied by FT-IR and XPS. Fig. 5 (c) presents the typical IR spectrum recorded on L-cysteine @ Fe2O3. For comparison, the IR spectrum of L-cysteine powders is also shown. According to earlier study on peak assignments of Lcysteine by IR [33], the peaks in the regions of 1580 cm1 and 1388 cm1 correspond to the asymmetric and symmetric carboxylate group (COO) stretching vibration. The presence of thiol group (SeH) can be proved by the peaks at 2550 cm1 and 940 cm1representing to its stretching vibration. The peaks at 1346 cm1 and 1140 cm1 can be assigned to the amino group (eNH2). Comparing with the spectrum of pure L-cysteine, the SeH vibration band is not detected for the specimen of L-cys @ Fe2O3 NTs. The absence of this band demonstrates that L-cysteine is bonded to the substrate through sulfur atom [34]. It has been reported that the relative intensities of the carboxylate bands can be used to infer the orientation of the carboxylate species according to the IR surface selection rule [35]. In this study, the relative intensities of asymmetric carboxylate stretching band is lower than the symmetric one, which presents obviously different relationship comparing to the L-cysteine powders. The corresponding change in the spectrum may indicate the coordination of the carboxylate group to the substrate. So FT-IR spectra analysis suggests that Lcysteine could bond to the Fe2O3 surface through thiol and carboxylate group. To further clarify this molecular bonding mechanism, quantum calculations were adopted. The electron density distribution of the HOMO (Fig. 5 (b)) can depict the nucleophile center of L-cysteine molecule. It could be observed that the electronic densities focus on the sulfur and oxygen atoms. So the nucleophile centers of the molecule are sulfur and oxygen atoms, which could keep bonding to the positively charged substrate under anodic polarization and illumination condition. Furthermore, the same sample was also studied by XPS shown in Fig. 6. From the survey spectra (Fig. 6 (a)), the existence of C, Fe, O, S and N are confirmed. The high-resolution C 1s XPS spectra (Fig. 6 (b)) of Fe2O3 NTs can be deconvoluted to two kinds of carbon species based on Gaussian-Lorentzian fitting: the adventitious carbon adsorbs on the surface of the sample (284.6 eV) and C]O bond (288.2 eV) is detected attributed to CO2 adsorption [36,37]. But for the high-resolution C 1s XPS spectra of L-cys @ Fe2O3 NTs and pure L-cysteine powder specimen, there are three carbon species, CeS (285.2 eV), CeN (286.1 eV) and COOH/COO (288.0 eV) [38,39]. And in the high resolution spectra of Fe 2p (Fig. 6 (c)), FeO (709.8 eV), Fe2O3 (710.6 eV) and FeOOH (711.5 eV) can be recognized for both samples [40,41]. FeS (709.2 eV) can also be detected on the surface of L-cys @ Fe2O3 NTs [39]. The high resolution spectra of O1s (Fig. 6 (d)) can be deconvoluted into three types, O2 (528.5 eV), OH (530.1 eV, represents the surface hydroxyl groups) and eCOOH (530.9 eV, carboxylic group), respectively [39,42e44]. As can be seen, L-cys @ Fe2O3 NTs surface presents a higher percentage of OH than that of Fe2O3 NTs. In Fig. 6 (e), the detected of S 2p peak implies the presence of sulfides on surface of L-cys @ Fe2O3 NTs: -S2-(161.8 eV), S2- 2 (163.5 eV) and S6þ (186.5 eV) [45,46]. The presented S6þ suggests a strong chemical bonding is formed on the sample surface. The high resolution of N1s spectra (shown in Fig. 6
91
(f) suggests that there is no distinct change for the group of NH2 after PEC measurements. So it can conclude that the adsorption of Lcysteine on Fe2O3 NTs is essentially stable even after PEC measurements. And it could highlight the surface state modification due to L-cysteine adsorption on Fe2O3 NTs surface. This observation is agreed with Somorjai etc., reported that the chemisorption of small molecules on the substrate can induce the surface restructuring [47]. In current experimental condition, the photoanode surface wettability has an important influence on PEC performance. The contact angle measurement is a method to characterize the surface wettability to some extent [48]. The measured contact angle images are shown in Fig. 7, which have been collected on L-cys @ Fe2O3 NTs and Fe2O3 NTs. It can be observed that the smaller average contact angle (7 ) appears on the surface of L-cys @ Fe2O3 NTs in comparison with Fe2O3 NTs sample (32 ), demonstrating its better wettability and much easier access of the aqueous electrolyte to the active sites [49]. It is well accepted that optical absorption of photoanode significantly influences their activity. The optical properties of L-cys @ Fe2O3 NTs and Fe2O3 NTs were estimated by using UVeVisible absorption spectroscopy. Fig. 8 shows the absorbance variations as a function of wavelength between 200 and 800 nm. It can be seen that L-cys @ Fe2O3 NTs has higher optical absorption than Fe2O3 NTs, both of which cover the UV and visible region. Based on the reported band assignments, the typical absorption bands appeared in region 250e400 nm are attributed to the ligand-to-meal charge transfer transitions with combined contributions from the Fe3þ ligand field transitions 6A1(6S) to 4T1(4P) at 290e310 nm, 6A1(6S) to 4 E (4D) and 6A1(6S) to 4T1 (4D) at 360e380 nm. The absorption bands in the region of 400e600 nm are due to the double excitation processes 6A1(6S)þ 6A1(6S) to 4T1 (4G)þ 4T1 (4G) at 480e550 nm, which could partly overlap with the ligand filed transition 6A1(6S) to 4E (4D) and 6A1(6S) to 4A1 (4G) at 430 nm [50,51]. Furthermore,
Fig. 8. UVeVisible absorption spectra of Fe2O3 NTs and L-cys @ Fe2O3 NTs specimen.
Fig. 7. Contact angle images of Fe2O3 NTs (a) and L-cys @ Fe2O3 (b) NTs.
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Fig. 9. Chronopotentiometry curves of L-cys @ Fe2O3 NTs and Fe2O3 NTs recorded at 10 mA/cm2 in 1 M NaOH media under dark; (b) Schematic model of the PEC water splitting occurred on L-cys @Fe2O3 NTs surface.
the band gap values have been determined from the spectra extrapolations: L-cys @ Fe2O3 NTs (1.99 eV) and Fe2O3 NTs (1.94 eV), which is consistent with the literature reports [6,51,52]. There is no significant band gap shift by L-cysteine deposition on Fe2O3 NTs surface. The absorption spectra results are consistent with improved photocurrent performance observed on L-cys @ Fe2O3 NTs sample. Chronopotentiometry measurement was performed at a constant current density of 10 mA/cm2 on L-cys @ Fe2O3 NTs and Fe2O3 NTs in 1 M NaOH solution under dark condition. The recorded curves are shown in Fig. 9 (a). Apparently, there is no obvious potential degradation observed on the L-cys @ Fe2O3 NTs over 5000 s continuous monitoring. But the potential of Fe2O3 NTs sample shifts about 74 mV to lower potential. Earlier studies have reported that water oxidation on hematite is mediated by the surface formation of iron complex [9,27,53,54]. The iron complex is unstable under high potential polarization. Hence, it's can be considered that Lcysteine acts as curing agent on the hematite surface. Hematite is one of the most prime candidates for photoanodes in PEC water splitting. The low visible light absorption coefficient and short hole migration length are the main drawbacks and limit its PEC performance. The main contribution of this work is to adopt a simple method on hematite photoanode fabrication, which helps to improve its main drawbacks. The fabricated Fe2O3 nanotubes are in well-ordered array. This one-dimensional nanostructure can facilitate hole migration. However, this is not sufficient for the separation of the pair. So the corresponding PEC activity is still limited. In considering this, we deposited a small quantity of Lcysteine on the nanostructured hematite surface. This amino molecule could chemical bonding on the substrate through thiol and carboxylate group. By adsorption, it can not only help hematite to resist corrosion in alkaline solution but also act as conductive linker. Under anodic polarization and illumination condition, the injection of hole could be effectively transferred to the electrolyte to promote water oxidation at the hematite surface. A model is proposed to illustrate the corresponding mechanism, as shown in Fig. 9 (b).
4. Conclusions In this study, we have prepared well arrayed hematite nanotube by electrochemical anodization on mild steel. When applying Lcysteine on the corresponding nanostructure, a potent behavior on PEC water splitting is observed. The enhanced performance is attributed to the modified surface states by chemical bonding of the nucleophile center including carboxylate and thiol group to the substrate. Then L-cysteine acts as a conductive linker and surface
curing agent to facilitate the flux of hole transfer. The highlight of this study is to supply a simple and effective method of L-cys @ Fe2O3 NTs photoanode preparation for PEC water splitting and attempt to explain the mechanism. Acknowledgments This work was supported by the National Natural Science Foundation of China [Grant No. 51301199] and the funding of China University of Petroleum (Beijing) [Grant No. 2462015YQ0602]. We are also grateful to the reviewers for the helpful comments. We also thanks Dr. Xueyuan Zhang (Gamry Instruments) for valuable suggestions. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37e38. [2] M.T. Mayer, C. Du, D. Wang, Hematite/Si nanowire dual-absorber system for photoelectrochemical water splitting at low applied potentials, J. Am. Chem. Soc. 134 (2012) 12406e12409. [3] G. Rahman, O.-S. Joo, Photoelectrochemical water splitting at nanostructured a-Fe2O3 electrodes, Int. J. Hydrogen Energy 37 (2012) 13989e13997. [4] S. Begonja, L.A.G. Rodenas, E.B. Borghi, P.J. Morando, Adsorption of cysteine on TiO2 at different pH values: surface complexes characterization by FTIR-ATR and Langmuir isotherms analysis, Colloids Surf. A Physicochem. Eng. Asp. 403 (2012) 114e120. pez, D. Monllor-Satoca, J.D. Prades, M.D. Hern [5] C. F abrega, S. Murcia-Lo andezAlonso, G. Penelas, J.R. Morante, T. Andreu, Efficient WO3 photoanodes fabricated by pulsed laser deposition for photoelectrochemical water splitting with high faradaic efficiency, Appl. Catal. B Environ. 189 (2016) 133e140. €tzel, Solar water splitting: progress using he[6] K. Sivula, F. Le Formal, M. Gra matite (a-Fe2O3) photoelectrodes, ChemSusChem 4 (2011) 432e449. [7] J.H. Kennedy, K.W. Frese, Photooxidation of water at a - Fe2 O 3 electrodes, J. Electrochem. Soc. 125 (1978) 709e714. [8] K.X. Wang, Z. Yu, V. Liu, M.L. Brongersma, T.F. Jaramillo, S. Fan, Nearly total solar absorption in ultrathin nanostructured iron oxide for efficient photoelectrochemical water splitting, ACS Photonics 1 (2014) 235e240. [9] B. Klahr, S. Gimenez, F. Fabregat-Santiago, T. Hamann, J. Bisquert, Water oxidation at hematite photoelectrodes: the role of surface states, J. Am. Chem. Soc. 134 (2012) 4294e4302. [10] H.K. Mulmudi, N. Mathews, X.C. Dou, L.F. Xi, S.S. Pramana, Y.M. Lam, S.G. Mhaisalkar, Controlled growth of hematite (a-Fe2O3) nanorod array on fluorine doped tin oxide: synthesis and photoelectrochemical properties, Electrochem. Commun. 13 (2011) 951e954. [11] A. Annamalai, P.S. Shinde, T.H. Jeon, H.H. Lee, H.G. Kim, W. Choi, J.S. Jang, Fabrication of superior a-Fe2O3 nanorod photoanodes through ex-situ Sndoping for solar water splitting, Sol. Energy Mater. Sol. Cells 144 (2016) 247e255. [12] Z. Zhang, M.F. Hossain, T. Takahashi, Self-assembled hematite (a-Fe2O3) nanotube arrays for photoelectrocatalytic degradation of azo dye under simulated solar light irradiation, Appl. Catal. B Environ. 95 (2010) 423e429. [13] F. Le Formal, N. Tetreault, M. Cornuz, T. Moehl, M. Gratzel, K. Sivula, Passivating surface states on water splitting hematite photoanodes with alumina overlayers, Chem. Sci. 2 (2011) 737e743. [14] Y. Lin, Y. Xu, M.T. Mayer, Z.I. Simpson, G. McMahon, S. Zhou, D. Wang, Growth of p-type hematite by atomic layer deposition and its utilization for improved
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