Article pubs.acs.org/JPCC
Efficient Photochemical, Thermal, and Electrochemical Water Oxidation Catalyzed by a Porous Iron-Based Oxide Derived Metal− Organic Framework Yingying Feng,† Jie Wei,† and Yong Ding*,†,‡ †
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China S Supporting Information *
ABSTRACT: Iron-based catalysts are of particular interest for water oxidation because of their high abundance, low toxicity, and rich redox properties. Herein, we report low cost porous iron-based oxides derived from calcining precursors of Prussian blue analogue (PBA) Mx[Fe(CN)6]y (M = Fe, Co, Ni). This synthesis approach involves a simple self-assembly technology and a low-temperature annealing procedure. These catalysts were investigated for photocatalytic, cerium(IV)-driven, and electrochemical water oxidation, and they exhibited superior activity. It is noteworthy that this photocatalytic water oxidation was conducted under neutral conditions that are similar to the natural photosystem II. The high initial turnover frequency (TOF) of ∼5.4 × 10−4 s−1 per transition metal atom at the first 60 s is obtained under neutral pH using porous CoxFe3−xO4 in photocatalytic water oxidation reaction, which is comparable with those published iron-based catalysts. Under cerium(IV)-driven water oxidation conditions, the TOF of porous CoxFe3−xO4 is 5.2 × 10−4 s−1 per transition metal atom, which is the highest value among all the documented iron oxides. In the electrochemical water oxidation, the porous NixFe3−xO4 catalyst exhibits a low overpotential of 402 mV at 10 mA cm−2. Meanwhile, the porous ironbased oxides possess beneficial ferromagnetic properties and excellent stability so that they were used repeatedly without loss in activity.
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INTRODUCTION Due to the increasing demand for energy and deteriorating environment, it has become an important challenge to develop clean and sustainable energy sources.1,2 Splitting of water is a cheap and efficient way to convert solar energy into chemical fuels. The four-electron process involving water oxidation (2H2O → 4H+ + 4e− + O2) is the rate-limiting step in the whole process.3 Therefore, exploiting abundant and robust water oxidation catalysts (WOCs) (Figure 1) is very significant to energy conversion. In the past decades, much efforts have concentrated on noble metal oxides of iridium 4 and ruthenium.5 However, their cost is exorbitantly high for widespread usage. To date, earth-abundant, eco-friendly, and inexpensive manganese oxides,6−15 iron oxides,16−23 cobalt oxides,24−34 and nickel oxides35−40 have been extensively explored and reported. Iron is the most abundant transition metal on Earth. The low price, low toxicity, and rich redox properties render iron-based catalysts highly favorable for water oxidation.41 Porous materials are attractive in heterogeneous catalysis because of their high surface areas, large pore volumes, narrow pore size distributions, and surface functionalities.42,43 Due to the high © 2015 American Chemical Society
surface energy, the traditional nanomaterials would agglomerate during the catalytic process, leading to the decline of catalytic activity. Jiao reported that nanostructured Co3O4 clusters supported on mesoporous silica superiorly catalyzed water oxidation because of their large specific surface areas.26 Mesoporous silica can prevent nanomaterials agglomeration and endow catalysts with high surface area. However, the preparation of nanostructured oxides in mesoporous silica by the hard template method involves a complicated and timeconsuming process that limits the large-scale utilization of these materials. Metal−organic frameworks (MOFs) represent an emerging type of materials that have attracted wide research due to their tunable porosities and versatile functionalities.44 Thermal calcination of MOFs could create porous structures with a relatively high surface area. Therefore, MOFs could be used as precursors and templates to prepare porous oxides as WOCs. On the basis of MOF-derived routes, the resulting ironbased oxides could inherit porous structure from MOFs and Received: November 25, 2015 Revised: December 16, 2015 Published: December 16, 2015 517
DOI: 10.1021/acs.jpcc.5b11533 J. Phys. Chem. C 2016, 120, 517−526
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and washed with deionized water, then finally dried in air at 60 °C. Ni3[Fe(CN)6]2 and Fe4[Fe(CN)6]3 precursors were prepared according to the literature. Porous iron oxides were obtained via calcination of precursors Mx[Fe(CN)6]y (M = Fe, Co, Ni) in air at 450 °C for 2 h with a slow heating rate of 1 °C min−1. Instrumental Characterization. Powder X-ray diffraction (PXRD) results were obtained with a PANalytical X’PertPro Diffractometer handled at 40 kV and 40 mA with Cu Kα radiation in the 2θ range of 10°−80°. Specific surface areas were computed from the N2 physisorption results collected at 77 K (Micromeritics ASAP 2020 M system) by employing the BET and BJH (Barrett−Joyner−Halenda) methods. The BET surface areas were estimated by adsorption data in a relative pressure range from 0.1 to 1. Field emission scanning electron microscopy (SEM) observations were performed on a Hitachi S-4800 microscope manipulated at an accelerating voltage of 5.0 kV. Transmission electron microscope (TEM) images were gained with a JEOL JEM-2010 instrument operated at 200 kV. X-ray photoelectron spectra (XPS) were measured by ESCALAB250xi with X-ray monochromatization. The highresolution XPS spectra were obtained for C 1s, O 1s, Fe 2p, Ni 2p, and Co 2p levels with pass energy 20 eV and step 0.1 eV. The binding energies were calibrated relative to the C 1s peak energy position as 284.8 eV. Infrared spectra were recorded using a Bruker VERTEX 70v FT-IR spectrometer. The results were recorded in the range of 400−4000 cm−1. Photocatalytic Oxygen Evolution from Water. Photoinduced water oxidation experiments were investigated by using the [Ru(bpy)3]2+−S2O82−−light system made of [Ru(bpy)3]Cl2 (1 mM), Na2S2O8 (5 mM), and 0.42 g L−1 catalysts in 12 mL of phosphate buffer (pH 7.0, 0.1 M). The solution was deaerated by purging with Ar gas for 8 min in the glass reactor (28 mL) sealed with a rubber septum. The reaction was then started by illuminating the solution with an LED light source (light intensity 16 mW, beam diameter 2 cm) through a transmitting glass filter (λ ≥ 420 nm) at room temperature. After each sampling time, 100 μL of Ar was injected into the reactor, and then the same volume of gas sample in the headspace of reactor was retracted by a SGE gastight syringe and analyzed by gas chromatography (GC). The total amount of generated O2 was calculated based on the concentration of O2 in the headspace gas. Isotope-labeled experiment was performed by repeating the experiment under the photocatalytic water oxidation conditions. The 10.8 atom % H218O of phosphate buffer (pH 7.0, 0.1 M) including porous iron-based oxides (0.42 g L−1), Na2S2O8 (5.0 mM) [Ru(bpy)3]Cl2 (1.0 mM), was deaerated with Ar gas in a flask sealed with a rubber septum before irradiation by LED light (λ ≥ 420 nm). After irradiation for 11 min, the resulting values of m/z 32 (O16O16), 34 (O16O18), and 36 (O18O18) were detected systematically by GC−MS. Cerium(IV)-Driven Oxygen Evolution from Water. Chemical oxygen evolution experiments were performed using aqueous solution with ceric ammonium nitrate (CAN) as the sacrificial one-electron acceptor. In a typical reaction, 11 mL of catalyst suspension (5 mg of catalyst was dispersed in 11 mL of deionized water) was put into the glass reactor (28 mL) which was then degassed by purging Ar gas continuously for the period of 8 min. An amount of 0.987 g of (NH4)2Ce(NO3)6 was dissolved in 1.5 mL of distilled water. This solution was deaerated by purging with Ar gas for 8 min. Anaerobic solution (1 mL) of 0.1 M CAN was then injected to the reactor to
Figure 1. Three technologies routinely used in evaluating wateroxidation activities of WOCs: (1) photochemical oxidation with the [Ru(bpy)3]2+−S2O82−−light system; (2) chemical oxidation using CAN; and (3) electrochemical oxidation.
display large surface area and narrow pore size distributions which are significant for excellent water oxidation performance. Herein, we report porous iron-based oxides as photocatalytic, cerium(IV)-driven, and electrocatalytic WOCs. These materials were derived from calcining precursors of Prussian blue analogue (PBA) Mx[Fe(CN)6]y (M = Fe, Co, Ni). PBAs are a type of crystalline MOFs built from divalent and trivalent metal ions linked by cyanide ligands. Our synthesis approach involves a simple self-assemble technology and a low-temperature annealing procedure. We conducted the photocatalytic water oxidation under neutral conditions that are similar to the natural photosystem II. These porous iron-based oxides exhibit superior activities for photocatalytic oxygen evolution. Under cerium(IV)-driven water oxidation conditions, the porous CoxFe3−xO4 gives the best activity among all the documented iron oxides. All the porous iron-based oxides show good electrocatalytic water oxidation performance.
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EXPERIMENTAL SECTION Materials. All chemical reagents (analytical grade) were used as received without any further purification. Purified water (18.2 MΩ.cm) for the preparation of solutions was attained from a Molecular Lab Water Purifier. H218O (97% 18O) was purchased from MASHALL ISOTOPES Ltd. Synthesis of Porous Iron-Based Oxides. Porous ironbased oxides were synthesized according to modified literature.45,46 In a typical synthesis, 100 mL of K3[Fe(CN)6]2 (2 mmol) was slowly added dropwise to 100 mL of aqueous solution containing Co(CH3COO)2 (1 mmol) and polyvinylpyrrolidone (PVP, K-30) (4 g) under stirring, and after 10 min a dark purple turbid liquid was formed and then aged for 24 h. The above reaction is executed at room temperature. The resulting product was collected by several centrifugations 518
DOI: 10.1021/acs.jpcc.5b11533 J. Phys. Chem. C 2016, 120, 517−526
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Figure 2. SEM images of (a) Fe4[Fe(CN)6]3, (b)Co3[Fe(CN)6]2, and (c) Ni3[Fe(CN)6]2 precursors. SEM images of (d) Fe2O3, (e) CoxFe3−xO4, and (f) NixFe3−xO4.
atoms uniformly dispersed in the Mx[Fe(CN)6]y (M = Fe, Co, Ni) can be oxidized into gases and escape in the annealing process, which finally results in interconnected porous surfaces. In comparison to other materials prepared via the hard template method, the advantages of our synthetic catalysts include (a) can gain low-cost materials on a large scale, (b) use a simple self-assemble and anneal procedure without any posttreatment, (c) can obtain the particular porous nature of nanomaterials to enhance surface area. The characterization of precursors and oxides was conducted by a number of techniques. First, the as-synthesized Mx[Fe(CN)6]y (M = Fe, Co, Ni) was characterized by PXRD (Figure S2), and the reflections gained can be assigned to Fe4[Fe(CN)6]3 (JCPDS 73-0687), Co3[Fe(CN)6]2 (JCPDS 86-0502), and Ni3[Fe(CN)6]2 (JCPDS 82-2283), respectively. These sharp peaks show that the as-prepared precursors have good crystallinity. The XRD patterns of iron-based oxides obtained by calcination of Mx[Fe(CN)6]y (M = Fe, Co, Ni) are shown in Figure S3, and the diffraction peaks belong to Fe2O3 (JCPDS 33-0664), CoxFe3−xO4 (JCPDS 22-1068), and NixFe3−xO4 (JCPDS 10-0325), respectively. The broadened peaks are due to the broadening effect of the small-size grains present in the samples.47 The FT-IR spectra of the samples are shown in Figures S4 and S5, and the broad absorption peaks at 3000− 3600 cm−1 are assigned to the stretching vibrations of the −OH group (absorbed water molecules).46 In Figure S4, the spikes from 2145 to 2166 cm−1 are attributed to CN stretching. These peaks disappear in the spectra of synthesized iron-based oxides
initiate the chemical water oxidation, and oxygen evolution was monitored by GC and similar to that of photocatalytic water oxidation. Electrochemical Oxygen Evolution Reaction. Electrocatalytic activities of the catalysts were tested in freshly prepared 1 M KOH (pH 14.0) solution with the standard three-electrode electrochemical glass flask using a CHI660D electrochemical analyzer. The working electrode was prepared by dispersing 2 mg of catalyst powder in 1 mL of ethanol and then 5 μL of solution slowly drop-cast onto a glassy carbon electrode (GCE, 3 mm in diameter). An amount of 2 μL of 0.5 wt % Nafion was cast on the top of the film to coat the catalyst to confirm the enhancement of mechanical stability of the electrodes. A Pt wire was used as the counter electrode and Ag/ AgCl (saturated KCl) as the reference electrode. The solution kept unstirred during the experiments. The potentials reported in this work were referenced to the reversible hydrogen electrode (RHE) through RHE calibration (in 1 M KOH at pH 14, ERHE = EAg/AgCl + 0.197 V + 0.059 pH).
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RESULTS AND DISCUSSION Synthesis and Characterization. Porous iron-based oxides were derived from calcining precursors of Prussian blue analogue (PBA) Mx[Fe(CN)6]y (M = Fe, Co, Ni). When Mx[Fe(CN)6]y (M = Fe, Co, Ni) precursors were annealed in air, the short distance between M and Fe could facilitate the formation of porous iron-based oxides with abundant nanometer-sized particles. Furthermore, a large number of C and N 519
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Figure 3. TEM images of (a) Fe2O3, (b) CoxFe3−xO4, and (c) NixFe3−xO4.
eV, respectively (Figure 4a). The spin−orbit splitting is 15.0 eV, revealing the coexistence of Co2+ and Co3+ species.49 After
(Figure S5), indicating those groups have decomposed after calcination. The broad peaks in the range of 500−700 cm−1 can be assigned to the metal−O vibrations (Figure S5). The morphology and structure of the precursors and catalysts were investigated by SEM. The SEM images of Figure 2a−c clearly display that these precursors have smooth facets. Figure 2d−f shows SEM images of as-prepared iron-based oxides, confirming that they preserve well the uniform size and morphology of the precursor particles. It is found that the surface of catalysts becomes rougher than that of precursors, which indicates the formation of abundant pores during the annealing procedure. The interior structure and porous nature of iron oxides were further investigated by TEM. Figure 3 exhibits the typical TEM images of the gained porous materials. A large number of pores (the light points on the surface) can be distinctly seen, which is consistent with what was observed from SEM. To obtain chemical composition, quantification of metal elements ratio, and their present state in the phase, ICPAES and EDX (Figure S6 and Table S1) were performed. The results show that metal to Fe ratio is 1.5:1 in MxFe3−xO4 (M = Co, Ni) materials. The formula of porous oxides can be expressed as M1.8Fe1.2O4 (M = Co, Ni). To further research the specific surface area and the porous feature of the as-prepared catalysts, BET gas-sorption measurements were performed. The BET specific surface areas are 104.8 m2/g for Fe2O3, 37.4 m2/g for CoxFe3−xO4, and 128.4 m2/g for NixFe3−xO4, respectively. These values are higher than other iron-based materials, which have been reported previously.16,18,19 The porosity of the annealed oxides was determined by N2 adsorption−desorption experiments (Figures S7, S8, and S9). The isotherm curves of the iron-based catalysts can be classified to a type IV curve with the H2-type hysteresis loop at the relative pressure of 0.4−0.9, thus implying the existence of a large amount of mesopores in the oxides.47 From the Barrett−Joyner−Halenda (BJH) pore-size distribution pattern, the samples of Fe2O3, CoxFe3−xO4, and NixFe3‑xO4 have a narrow pore width distribution at about 5, 3, and 4 nm, respectively. In addition, the single peak with narrow size distribution inset in Figures S7, S8, and S9 verifies that the asprepared iron-based oxides have uniform pore structure, which is in good approval with the results determined by TEM observations. The detailed chemical valence states of iron oxides were further characterized by XPS experiment. For Fe2O3 porous oxide, the peaks of Fe 2p1/2 and Fe 2p3/2 are located at 724.3 and 710.7 eV, accompanied by two shakeup satellite peaks (718.3 and 732.2 eV) (Figure S10a). After refined fitting, the appearance of two peaks at 710.4 and 723.8 eV is attributed to Fe(II), whereas 712.7 and 725.8 eV can be assigned to the Fe(III) cation.48 The Co 2p spectrum of CoxFe3−xO4 displays two major peaks for Co 2p1/2 and Co 2p3/2 at 795.2 and 780.2
Figure 4. XPS spectra of CoxFe3−xO4: (a) Co 2p, (b) Fe 2p, and (c) O 1s.
fitting, the peaks of 780.0 and 795.1 eV are characteristic for Co(III), and the peaks at 781.8 and 796.5 eV are for Co(II). The XPS spectrum of Ni 2p3/2 and Ni 2p1/2 for NixFe3−xO4 oxide exhibits peaks at 854.9 and 872.8 eV (Figure S11a),50 indicating that the Ni exists in a range of typical Ni2+ or Ni3+. 520
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solid catalysts, we investigated recycled CoxFe3−xO4 under photocatalytic water oxidation conditions. The solid catalyst can be easily recovered by magnet because of the strong ferromagnetic properties of the iron-based oxides (Figure S16b). A fresh phosphate buffer solution containing [Ru(bpy)3]Cl2 (1.0 mM) and Na2S2O8 (5.0 mM) was added to the recovered solid catalyst for the repetitive examinations under illumination. The high catalytic activity of iron oxides was maintained well even for the fourth run (Figure 6). The porous
The strong satellite peaks show that most of the nickel elements in the crystal lattice are Ni2+ species.35,51 The O 1s spectra for all oxides were fitted to the two peaks that correspond to metal oxygen bonds in the metal oxides and OH− (hydroxyl-like group) (Figures 4c, S10b, and S11c).49 Photocatalytic Water Oxidation Activities of Porous Iron Oxides. The photocatalytic water oxidation for the porous iron-based samples was performed in phosphate buffer solution of pH 7.0 in the presence of a two-electron acceptor (Na2S2O8) and photosensitizer ([Ru(bpy)3]Cl2). The choice of neutral conditions is important because nature provides the same conditions in the oxygen-evolution system (photosystem II). To confirm the source of oxygen, the oxidation reaction was characterized in 10.8 atom % H218O of phosphate buffer (pH 7.0, 0.1 M) (Figure S12). The data clearly demonstrate that the source of oxygen evolved is water. A series of experiments which involve the absence of catalyst, [Ru(bpy)3]Cl2, and Na2S2O8 were carried out, and no O2 evolution was detected (Figure 5). These results prove that each of the above
Figure 6. Time courses of O2 formation in a phosphate buffer solution containing [Ru(bpy)3]Cl2 (1.0 mM) and Na2S2O8 (5.0 mM) and CoxFe3−xO4 (5 mg) under photoirradiation (LED lamp, λ ≥ 420 nm) at room temperature in four repetitive examinations.
CoxFe3−xO4 catalysts were investigated before and after the reaction by XRD (Figure 7) and TEM (Figure 8) measure-
Figure 5. Photocatalytic O2 evolution in a phosphate buffer solution (pH 7.0, 12.0 mL) with Na2S2O8 (5.0 mM), [Ru(bpy)3]Cl2 (1.0 mM), and catalysts (5 mg) (LED lamp, λ ≥ 420 nm) at room temperature.
components is essential for the photocatalytic water oxidation system. Compared with commercial Fe2O3, all the porous ironbased materials exhibit obvious photocatalytic activities (Figure 5). The amount of O2 evolution over CoxFe3−xO4 after 13 min illumination was 9.5 μmol, which was significantly higher than that with NixFe3−xO4 (6.4 μmol) and Fe2O3 (2.3 μmol). The sacrificial electron acceptor of Na2S2O8 was used up, and the photosensitizer of [Ru(bpy)3]Cl2 was decomposed during the photocatalytic process, so that the amount of O2 formation is limited. The oxygen yields of all catalysts have been normalized to per mole of transition metal. After 60 s photoirradiation, the oxygen evolution rates of all catalysts were obtained (Figure S14 and Table S2). The highest rate was obtained over the CoxFe3−xO4 with a value of 0.54 mmolO2 molmetal−1 s−1 that was 2.5 and 6.3 times higher than that of NixFe3−xO4 (0.22 mmolO2 molmetal−1 s−1) and Fe2O3 (0.085 mmolO2 molmetal−1 s−1), respectively. Moreover, the catalytic activities of samples were compared by BET surface area normalization (Figure S15 and Table S2). The results disclose the value for CoxFe3−xO4 (0.18 μmol s−1 m−2) is superior to that of NixFe3−xO4 (0.02 μmol s−1 m−2) and Fe2O3 (0.01 μmol s−1 m−2). Stability Studies of Porous Iron Oxides for Photocatalytic Water Oxidation. To evaluate the stability of the
Figure 7. Powder XRD patterns of fresh porous CoxFe3−xO4 (red) and recovered porous CoxFe3−xO4 after photocatalytic reaction (black).
ments. No obvious change in either XRD patterns or the morphology of the CoxFe3−xO4 catalysts is observed. HR-TEM images for recovered porous CoxFe3−xO4 indicate that there is no visible amorphous layer formation on the surface or detectable dissolution from the surface (Figure S17). The surface conditions of porous CoxFe3−xO4 before and after the photocatalytic reaction were observed by XPS, which was performed in the Co 2p, Fe 2p, and O 1s energy regions. The Co 2p spectrum for the recovered catalyst is shown in Figure S18a. The consistency of peak shapes and similar intensity ratios for the main and satellite peaks for the samples of before and after the reaction indicates that the surface conditions are the same after the photocatalytic water 521
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Figure 8. TEM images of CoxFe3−xO4 (a) before the photocatalytic water oxidation and (b) after photocatalytic water oxidation.
The porous CoxFe3−xO4 catalyst was gathered after cerium(IV)-driven water oxidation by a magnet due to its ferromagnetic properties (Figure S16c). The catalytic activity of CoxFe3−xO4 was retained well for the fourth cycle (Figure S21). No significant variation in the total amount of O2 evolution was observed in the third and fourth runs under cerium(IV)-driven water oxidation reaction. To further confirm the stability of porous CoxFe3−xO4 during cerium(IV)-driven water oxidation, the powder XRD (Figure S22) and TEM (Figure S23) after the reaction were conducted. No obvious alteration in either powder XRD patterns or the TEM images is observed, indicating that porous CoxFe3−xO4 is stable. Electrochemical Water Oxidation. Electrochemical water oxidation experiments were measured in alkaline 1 M KOH solution using linear sweep voltammetry (LSV) at a scan rate of 10 mV s−1. The excellent oxygen evolution catalysts are required to expedite the reaction rate and reduce the overpotential. As shown in Figure 10, for NixFe3−xO4 porous
oxidation. The Fe 2p spectrum in Figure S18b exhibits a slight increase in Fe3+ character from the deconvolution area, which shows that a few Fe2+ ions have been oxidized to Fe3+ ions. The corresponding O 1s spectrum is given in Figure S18c. The peak at 530.2 eV can be ascribed to the lattice oxygen of CoxFe3−xO4, whereas a large dominance of OH− species absorbed on the surface are observed at the binding energy of 532.2 eV. XPS quantitative analysis from the sample after the reaction manifests that the atomic ratio of Co/Fe is the same as that of fresh catalyst. Thus, these results clearly indicate that porous CoxFe3−xO4 is a highly active and stable catalyst in the photocatalytic system. Cerium(IV)-Driven Water Oxidation. Cerium(IV)-driven water oxidation experiments were executed with catalysts in deoxygenated aqueous solution of 0.1 M ceric ammonium nitrate (CAN) (Figure 9). The porous CoxFe3−xO4 showed
Figure 9. Time courses of O2 evolution by using 5 mg catalysts in deoxygenated aqueous solutions with 0.1 M CAN as an oxidant. Figure 10. Linear sweep voltammetry (LSVs) (sweep rate 10 mV s−1) with a carbon paste-working electrode (A = 0.071 cm2) containing all catalysts in 1 M KOH at room temperature.
superior activities for cerium(IV)-driven water oxidation. After 45 min reaction, CoxFe3−xO4 afforded 42.5 μmol O2, which was remarkably larger than that of NixFe3−xO4 (4.8 μmol) and Fe2O3 (2.5 μmol). The rate of oxygen generation was calculated for the first 60 s. The catalyst of CoxFe3−xO4 was detected to be exceedingly active with a maximum rate of 0.52 mmolO2 molmetal−1 s−1 considering all the active metal atoms. This rate was approximately 15-fold higher than that of NixFe3−xO4 (0.03 mmolO2 molmetal−1 s−1) and Fe2O3 (0.013 mmolO2 molmetal−1 s−1) (Figure S19 and Table S3). Meanwhile, surface area normalized curves are displayed in Figure S20, and the correlation follows the same order as that of total mass activity (Table S3), which agrees with the results of photocatalytic water oxidation.
materials, the electrocatalytic oxygen evolution starts at 1.52 V (vs the reversible hydrogen electrode, RHE). Similarly, for the CoxFe3−xO4 and Fe2O3, the currents start increasing at 1.54 and 1.61 V, respectively. The overpotential at 10 mA cm−2 is in general a key parameter to evaluate oxygen evolution catalysts.52 The overpotential for NixFe3−xO4 is 402 mV, whereas CoxFe3−xO4 and Fe2O3 show slightly higher overpotential with the values of 428 and 511 mV, respectively (Table S4). By plotting overpotential (η) against log (J/mA cm−2), the kinetic parameters of electrochemical water 522
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Figure 11. Tafel plots of the catalysts in 1 M KOH at pH 14.
oxidation over these catalysts were calculated (Figure 11). The tafel slope of NixFe3−xO4 is 53 mV dec−1, which is lower than that of CoxFe3−xO4 (57 mV dec−1) and Fe2O3 (68 mV dec−1). Tafel slopes are often influenced by electron and mass transport and could be used to investigate the mechanism of electrochemical oxygen evolution.53 Thus, the lower tafel slope of NixFe3−xO4 might be due to faster electron transport on the samples surface. The electrochemical characterization results indicate that NixFe3−xO4 has an as efficient eletrocatalytic activity as having higher current density, lower overpotential, and smaller tafel slope. To determine the catalytic stability of the porous iron-based oxides, the bulk electrolysis experiments were implemented. The bulk electrolysis of NixFe3−xO4 over 5000 s was recorded at 1.56 V vs RHE. The black plateau line in Figure 12 shows that porous NixFe3−xO4 is stable during the electrochemical water oxidation. The efficient water oxidation activity of iron-based oxides could be attributed to the porous nature and large surface area of the materials. In addition, based on the superior activity of porous iron-based oxides for photocatalytic and cerium(IV)driven water oxidation, a structure activity correlation could be deduced (CoxFe3−xO4 > NixFe3−xO4 > Fe2O3). The crystal structures of CoxFe3−xO4 and NixFe3−xO4 are cubic system and considered as spinel type (AB2O4) structures (Figure 13). There are two distinct sites in spinel structure: four-oxygen coordinated tetrahedral sites and six-oxygen coordinated octahedral sites. It has been well-known that the octahedral sites in a spinel structure play a key role compared to that of tetrahedral sites for the activity of water oxidation.24,49,54,55 The octahedral cobalt atoms and nickel atoms in CoxFe3−xO4 and NixFe3−xO4 might be crucial for an efficient oxygen evolution process. Moreover, the photocatalytic and cerium(IV)-driven water oxidation results demonstrate the negative effects of
Figure 12. Bulk electrolysis curve with a carbon paste-working electrode (A = 0.071 cm2) containing NixFe3−xO4 at 1.56 V vs RHE in 1 M KOH at room temperature.
transition metal Ni and Fe on catalyst activity, revealing that octahedral Co3+ in the CoxFe3−xO4 offers optimal binding energy for the intermediate formation on the catalyst surface. The results are in good agreement with those reported by Jiao and co-workers.54,55 Therefore, the excellent activity of CoxFe3−xO4 is ascribed to the highly efficient active sites created by Co cations in octahedral sites. Under electrochemical water oxidation conditions, the NixFe3−xO4 exhibits efficient activity (NixFe3−xO4 > CoxFe3−xO4 > Fe2O3). This might be becuase the Ni cations in the octahedral sites of the spinel structure increase active sites with much lower activation potential compared to that of the Co cation.37,56,57 It can be concluded that CoxFe3−xO4 and NixFe3−xO4 are efficient for water oxidation due to a large number of Co, Ni active sites. Therefore, the obtained results from the porous iron-based 523
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oxides describe the importance of morphological and structural properties.
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CONCLUSIONS In summary, we synthesized a series of iron-based porous nanomaterials used as water oxidation catalysts. These porous materials derived from the thermal decomposition of PBA Mx[Fe(CN)6]y·nH2O (M = Fe, Co, Ni) precursors. These ironbased materials were investigated for their water oxidation catalytic performance under photocatalytic, thermal, and electrocatalytic conditions. The photocatalytic water oxidation and cerium(IV)-driven water oxidation follow the same order as CoxFe3−xO4 > NixFe3−xO4 > Fe2O3, whereas the electrocatalytic water oxidation follows the order of NixFe3−xO4 > CoxFe3−xO4 > Fe2O3. Compared with other iron-based materials reported, water oxidation perfomance of our porous materials is more excellent. The strategy of utilization precursor opens a new route for preparing porous materials. Importantly, the porous materials are easily accessible even on a large to commercial scale. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11533. Contains XRD data, IR data, XPS data, N2 adsorption/ desorption BET data, and EDX data
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Figure 13. Polyhedral schematic representation of MxFe3−xO4 (M = Co, Ni). Turquoise: Co/Ni/Fe, red: O.
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21173105 and 21172098). The authors would like to thank Professor Shishan Sheng for helpful discussions about XPS. 524
DOI: 10.1021/acs.jpcc.5b11533 J. Phys. Chem. C 2016, 120, 517−526
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