Lateral-Size-Mediated Efficient Oxygen Evolution Reaction: Insights

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Lateral Size-Mediated Efficient Oxygen Evolution Reaction: Insights into the Atomically Thin Quantum Dot Structure of NiFe2O4 Haidong Yang, Yang Liu, Sha Luo, Ziming Zhao, Xiang Wang, Yutong Luo, Zhixiu Wang, Jun Jin, and Jiantai Ma ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00007 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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Lateral Size-Mediated Efficient Oxygen Evolution Reaction: Insights into the Atomically Thin Quantum Dot Structure of NiFe2O4 Haidong Yang†, Yang Liu†, Sha Luo†, Ziming Zhao†, Xiang Wang†, Yutong Luo†, Zhixiu Wang‡, Jun Jin*,† and Jiantai Ma*,† †

State Key Laboratory of Applied Organic Chemistry (SKLAOC), The Key Laboratory of

Catalytic Engineering of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu, 730000, P. R. China. ‡

State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical

Chemistry, Jilin University, Changchun 130023, P. R. China.

ABSTRACT The study of high-performance electrocatalysts for driving the oxygen evolution reaction (OER) is important for energy storage and conversion systems. As a representative of inverse spinel-structured oxide catalysts, nickel ferrite (NiFe2O4) has recently gained interest because of its earth abundance and environmental friendliness. However, the gained electrocatalytic performance of NiFe2O4 for the OER is still far from the state-of-the-art requirements because of its poor reactivity and finite number of surface active sites. Here, we

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prepared a series of atomically thin NiFe2O4 catalysts with different lateral sizes through a mild and controllable method. We found that the atomically thin NiFe2O4 quantum dots (AT NiFe2O4 QDs) show the highest OER performance with a current density of 10 mA cm-2 at a low overpotential of 262 mV and a small Tafel slope of 37 mV decade-1. The outstanding OER performance of AT NiFe2O4 QDs is even comparable to the commercial RuO2 catalyst, which can be attributed to its high reactivity and high fraction of active edge sites resulting from the synergetic effect between atomically thin thickness and small lateral size of the atomically thin quantum dot (AT QD) structural motif. The experimental results reveal a negative correlation between lateral size and OER performance in alkaline media. Specifically speaking, the number of low-coordinated oxygen atoms increases with decreasing lateral size, and this leads to significantly more oxygen vacancies that can lower the adsorption energy of H2O, increasing the AT NiFe2O4 QDs catalytic OER efficiency.

KEYWORDS water electrolysis, oxygen evolution reaction, nickel ferrite, atomically thin quantum dots, catalytically active sites. 1. INTRODUCTION

The oxygen evolution reaction (OER) bears great implications for the pursuit of alternative energy conversion technologies, for instance, water splitting and regenerative fuel cells.1,2 However, the performance of this electrochemical process is limited by the intrinsically sluggish kinetics of the OER because of the complex four-electron redox processes, and this hinders its large-scale application to these alternative energy technologies.3,4 To resolve this intrinsic kinetics limitation, the exploration of highly active electrocatalysts for OER is desperately needed. However, the development of OER electrocatalysts based on noble metals, such as RuO2


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and IrO2, are intensively impeded due to their scarcity and high cost. For this reason, a massive research effort has been undertaken towards exploring non-noble metals and their derivatives as OER electrocatalysts. Thus, a number of earth-abundant transition metal sulfides,5,6 nitrides,7,8 phosphides,7 oxides,9,10 hydroxides,11,12 and metal-free electrocatalysts13 have been explored as classical materials for OER. Among these materials include those based on spinel-structured oxides (AB2O4, A, B = metal), which consist of a closely packed array of O2- ions with the occupancy of A2+ and B3+ cations in tetrahedral and octahedral sites, respectively.14-16 Because of their outstanding properties, such as robust reactivity, relatively high electrical conductivity, and superior stability, NiCo2O4 and CoFe2O4 have been explored as attractive electrocatalysts for OER in previous reports.17-20 Notably, as a representative of an inverse spinel-structured oxide, nickel ferrite (NiFe2O4) has gained particular interest for electric/electronic devices and catalysis because of its earth abundance, environmental friendliness, high electrical conductivity, and long-term stability;21-24 however, the promise of its application as a catalyst for the highly efficient OER is still an open question. Recently, a three-dimensional ordered mesoporous NiFe2O4 with tuneable pore size for OER in Li-O2 batteries was achieved by Yang and his coworkers.22 Li et al. have successfully synthesized a one-dimensional NiFe2O4 as an electrode material for the OER.24 Liu et al. have reported that combining NiFe2O4 nanoparticles with Ni(OH)2 nanosheets can lead to an increased OER activity.21 Unfortunately, although progress has been made, the gained electrocatalytic performance of NiFe2O4 towards the OER is still far from the state-of-the-art requirements because of the poor reactivity and a finite number of active surface sites, as supported by the experimental measurements in previous studies.15,25 Consequently, an elaborate design of NiFe2O4 with an optimized structure for achieving the


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greatly enhanced OER electrocatalytic performance is still an enormous challenge and of particular interest. Active sites play an important role in the surface catalytic reaction;26-28 thus, increasing the intrinsic reactivity and number of active sites could dramatically enhance the OER electrocatalytic activity.29,30 On the one hand, the active sites should exhibit a high intrinsic reactivity, which is related to a preferential occupancy of low-coordinated atoms, determined by the chemical composition and surface structure of the catalyst.26,31-35 As previously mentioned, the spinel-structured AB2O4 contains a certain number of cations existing in both A and B sites.14,36 Based on previous experimental measurements and DFT calculations, the lowcoordinated B-site atoms at the surface of AB2O4, which can significantly improve the physical/chemical properties and enhance catalytic reactivity,14,37 can serve as active sites because a preferential occupancy of low-coordinated atoms could give rise to an increase in the number of neighbouring oxygen vacancies that are extremely beneficial for facilitating the transport of oxygen ions.17,38,39 On the other hand, the active sites should be accessible to the OER relevant species (e.g., OH-, O2), and this is determined by the superior electrotransport property originating from the electrochemically active surface area (Aechem), that is, the number of active sites.17,40 Thus, an effective spinel-structured OER electrocatalyst is expected to augment the number of active sites, that is, an abundance of oxygen vacancies, making it ideally suited for achieving a desirable electrocatalytic performance. It is worth mentioning that atomically thin two-dimensional (2D) nanosheets have been increasingly recognized as a promising platform to realize an improved catalytic activity, and such findings give us a deeper understanding of the relationship between active sites and catalysis on the atomic scale.35,41-43 Two-dimensional nanosheets, such as ultrathin NiCo2O4,


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Co3O4, and brucite-like metal-hydroxyl host materials, can clearly increase the number of lowcoordinated atoms as the material thickness is decreased.17,32,40 Driven by the synergetic effect between prominent edges and quantum confinement, with the lateral size of the 2D material being reduced to typically < 20 nm (atomically thin quantum dots, denoted as AT QDs), several improved and new properties appear, such as a higher ratio of surface area to volume, more exposed active sites, and in particular, an increase in the edge length with low-coordinated atoms. There is no denying that all these characteristics can lead to a highly efficient OER electrocatalytic activity.44-48 Inspired by the aforementioned considerations, we note that for NiFe2O4, exploring atomically thin NiFe2O4 quantum dots (AT NiFe2O4 QDs) with an atomic thickness and small lateral size to maximize the number of low-coordinated Fe-site atoms that can serve as the active sites would lead to an outstanding OER electrocatalytic activity. Herein, we report a simple yet controllable method via the template-induced in situ growth of atomically thin NiFe2O4 nanosheets on the surface of polystyrene (PS) spheres in a radial orientation, where the PS was employed as a template,49,50 and then, a calcination protocol was followed to separate the NiFe2O4 nanosheets from the template. By tailoring the NiFe2O4 catalysts, the experimental measurements confirm that a series of atomically thin NiFe2O4 nanosheets with different lateral sizes were synthesized. Meanwhile, a negative correlation between lateral size and OER performance in alkaline media was observed. DFT calculations demonstrate that the number of low-coordinated oxygen atoms increase with decreasing lateral size, and this leads to an increased presence of oxygen vacancies; this can lower the adsorption energy of H2O, yielding the AT NiFe2O4 QD catalyst an increased OER efficiency. The as-synthesized AT NiFe2O4 QDs with ~0.8 nm in thickness and ~8 nm in lateral size exhibit a high fraction of exposed active edge sites with low-coordinated atoms as


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well as a unique AT QD open-framework that can supply an ideal platform with an enlarged number of active sites, an enhanced reactivity, and optimized reaction kinetics. For this study, we provide insights into the NiFe2O4 that was traditionally regarded as a non-ideal catalyst for the OER, and the AT QD structural motif we present here endows NiFe2O4 with an abundance of substantially more reactive edge sites, making it ideally suited as a highly efficient OER electrocatalyst in basic media, simultaneously holding great potential for various applications such as energy storage and conversion systems. 2. EXPERIMENTAL SECTION 2.1 Synthesis of NiFe2O4/PS and AT NiFe2O4 QDs Specifically, the synthesis of the NiFe2O4 nanosheets can be divided into two parts, the “in situ growth process” and “separation process” (Figure S1). For the “in situ growth process” of NiFe2O4 nanosheets on PS, 1 mg mL-1 PS particles (prepared according to the method of soapfree emulsion polymerization50) with diameters of 300, 1000, and 2000 nm was dispersed into a aqueous sulfate electrolyte (pH = 6, 250 g L-1 NiSO4·6H2O, 25.6 g L-1 FeSO4·7H2O, and 5 g L-1 H3BO3 (Sinopharm Chemical Reagent Co., Ltd.)) for 30 min by ultrasonic treatment.51,52 After a 20 h rest period, the resulting solution was used as the electrolyte for electrodeposition. The electrodeposition process was operated in a conventional three-electrode electrochemical cell with an Ag/AgCl (sat. KCl) as the reference electrode, a copper foil (CF) as the working electrode, and graphite as the counter electrode. The CF with a geometric area of 1.28 cm2 (2 (sides) × 8 mm length × 8.0 mm width) and an electrochemical workstation (CHI 660E) were used for this purpose. The electrodeposition process was implemented by imposing a constant deposition potential (-2.6 V vs Ag/AgCl) for 2 h under ultrasonic treatment at 60 °C. Subsequently, the obtained electrolyte was sonicated for a total of 50 h. In a typical “separation


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process” of NiFe2O4 nanosheets from the PS template, a porcelain boat containing the homogeneous NiFe2O4/PS mixture was placed at the centre of a quartz tube and was heated to a temperature of 500 °C at a rate of 2 °C min-1 for 6 h with a N2 gas flow rate of ~25 mL min-1. During this calcining process, the PS template was transformed into the styrene monomer and other by-products upon pyrolysis, and the NiFe2O4 nanosheets were then separated from the original PS template. The nanosheets were cooled to room temperature naturally and were dispersed in ethanol. Subsequently, the samples were centrifuged for 5 minutes at 3000 rpm to collect the supernatant. The collected supernatant was then centrifuged for 8 minutes at 8000 rpm to extract the sediments. The extracted sediments were washed with deionized water (> 18 MΩ cm resistivity, Barnstead E-Pure system) to remove impurities and then dried in a vacuum oven, resulting in the final product of NiFe2O4 nanosheets. 2.2 Electrochemical Measurements To prepare the working electrode, the as-synthesized NiFe2O4 samples (5 mg) were dispersed in a 1 mL water–ethanol mixture (v/v = 1:1), and a slight ultrasonic treatment was carried out to obtain a homogenous catalyst ink. After a glassy carbon electrode (GCE, 3.0 mm diameter) was polished, 3 µL ink containing 15 µg catalysts was drop-casted on it. The catalyst loading amount was 0.21 mg cm-2. Then, an aqueous Nafion solution (2 µL, 0.5 wt%) was drop-casted onto the GCE and fully dried as a conductive binder. Electrochemical measurements were performed using an electrochemical workstation (CHI 660E) in alkaline media. A calibrated Hg/HgO electrode was used as a reference electrode, and a Pt wire was used as the counter electrode. In this paper, all potentials, first measured vs a Hg/HgO reference electrode, were then referenced vs an relative hydrogen electrode (RHE) by adding a value of Evs Hg/HgO + 0.059 × pH. For LSV


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measurements, the scan rate was 10 mV s-1. Additional details on the experimental and calculation methods are listed in the supporting information (SI). 3. RESULTS AND DISCUSSION The structural characteristics of as–synthesized AT NiFe2O4 QDs obtained from X-ray diffraction (XRD) are shown in Figure 1a. The diffraction peaks at 2θ values of 30.3°, 35.7°, and 43.4° are well indexed to the (220), (311), and (400) crystal planes of spinel-structured NiFe2O4 (JCPDS Card No.10-0325; Space group: Fd-3m; a = b = c = 8.339 Å), respectively, with no detectable impurities. Meanwhile, the XRD pattern demonstrates that the preferential growth orientation of the nanosheets is [400].17 XPS analysis was used to analyse the oxidation states of surface elements and to simultaneously identify the chemical composition of AT NiFe2O4 QDs. In the Ni 2p core-level spectrum (Figure 1b), the peak at 854.7 eV and its satellite peak at 861.7 eV are ascribed to Ni2+, while the binding energy peak at 855.9 eV is assigned to Ni3+.17 The peaks at 709.3 and 710.5 eV in Fe 2p region are ascribed to Fe2+ and Fe3+ (Figure 1c), respectively.53-56 Additionally, the calculated Ni2+/Ni3+ and Fe2+/Fe3+ ratios based on the area intensity are 3.11 and 0.51, respectively. Combined with the ICP-OES results, the actual molecular formula of AT NiFe2O4 QDs is Ni1.02Fe2O3.81, suggesting the existence of oxygen vacancies. For the O 1s region in Figure 1d, three peaks can be clearly distinguished. More specifically, the peak at 529.7 eV is associated with the chemical bonding between oxygen atoms and metal atoms (O1);17,57 the peak with a binding energy of 532.6 is due to the surface-adsorbed hydroxyl groups of H2O molecules (O2);58 and the peak at 531.2 eV can be assigned to the low oxygen coordinated defects sites (i.e., oxygen vacancies, O3), further indicating that an abundance of oxygen vacancies exist in as-synthesized AT NiFe2O4 QDs.17,59


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Figure 1. (a) XRD pattern of AT NiFe2O4 QDs taken after synthesis. (b) Ni 2p, (c) Fe 2p and (d) O 1s XPS spectra of AT NiFe2O4 QDs taken after synthesis. TEM was used to further characterize the morphology and microstructural information of AT NiFe2O4 QDs/PS and the final product AT NiFe2O4 QDs. The TEM images shown in Figure 2a-c indicate that the numerous individual AT NiFe2O4 QDs grow in situ on PS in a radial orientation, exhibiting a transparent morphology and a lateral size of approximately 15 nm. After being separated from the PS template through a calcination process, numerous individual transparent AT NiFe2O4 QDs only 8~10 nm in lateral size were collected (Figure 2d); this shows a unique structural flexibility and a sharper increase in both the exposed surface and active edge sites with


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Figure 2. (a-c) TEM images of AT NiFe2O4 QDs/PS. (d) An HRTEM image of several individual AT NiFe2O4 QDs. (e) An HRTEM image of a single AT NiFe2O4 QDs (inset: the SED pattern of the AT NiFe2O4 QD). (f) A Schematic representation of the crystal structure of the spinel-structured NiFe2O4. (g) An AFM image and (h) the corresponding height profile of assynthesized AT NiFe2O4 QDs. (i) A photograph of a colloidal suspension of as-synthesized AT NiFe2O4 QDs. low-coordinated atoms when the total number of atoms is kept constant. Meanwhile, this unique AT QDs architecture creates considerable contact between the active sites of AT NiFe2O4 QDs


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and the electrolyte. HRTEM images and the corresponding fast Fourier transform (FFT) pattern (Figure 2d, e and inset) clearly reveal that the nanosheets are single crystals with a lattice spacing of 0.209 nm corresponding to the (400) plane of AT NiFe2O4 QDs. Combined with the XRD results, we can conclude that the surface and edge exposed facets of AT NiFe2O4 QDs are both {400} (Figure 2f). The thickness of the as-synthesized AT NiFe2O4 QDs was measured using atomic force microscopy (AFM). The average scanning heights of AT NiFe2O4 QDs are in the range of 0.8~0.9 nm, indicating that the nanosheets mainly consist of approximately 1 unit cell (a single NiFe2O4 unit cell along the [400] direction is 0.83 nm), as shown in Figure 2g and h. To clarify the elemental composition of AT NiFe2O4 QDs, ICP-OES and XPS measurements were conducted. In Table S1, according to the measurements, the mean ratio of nickel to iron matches the theoretical value of NiFe2O4 (Ni/FeICP = 0.51, Ni/Fexps = 0.57, and Ni/Fetheo = 0.50) very closely, demonstrating that the AT NiFe2O4 QDs are only composed of nickel, iron, and oxygen elements. Furthermore, as shown in Figure 2i, a clear Tyndall light scattering of the colloidal suspensions of as-synthesized AT NiFe2O4 QDs was observed, further confirming the AT QD structure of the as-synthesized AT NiFe2O4 QDs. Together, these results indicate that the AT NiFe2O4 QDs with an atomically thin thickness and small lateral size were successfully prepared. To investigate the OER electrocatalytic performance, we obtained a uniform catalyst film of the as-synthesized AT NiFe2O4 QDs by drop-casting onto a GCE; the loading amount of the catalyst was 0.21 mg cm-2. Then, linear sweep voltammetry (LSV) measurements were carried out to estimate the OER activity of AT NiFe2O4 QDs in comparison with AT NiFe2O4 QDs/PS, Ni-Fe layered double hydroxides (Ni-Fe LDHs), CoFe2O4, and RuO2 samples. The LSV curves of these samples in Figure 3a (the current density was calculated on the basis of the geometric area of the GCE) provide evidence of the negligible OER activity of AT NiFe2O4 QDs/PS


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Figure 3. (a) LSV curves and (b) corresponding Tafel plots for AT NiFe2O4 QDs, AT NiFe2O4 QDs/PS, Ni-Fe LDHs, CoFe2O4, and RuO2 for the OER in 1 M KOH. (c) A comparison of the overpotential at current densities of 10 and 30 mA cm-2. (d) The electrocatalytic efficiency of H2 production over AT NiFe2O4 QDs (electrode area: 0.07065 cm2) at a current density of 20 mA cm-2 (measured for 200 min). Current densities are normalized with the geometric surface areas of the electrodes. because of the poor electrical conductivity originating from the organic PS insulator. It is important to highlight that the AT NiFe2O4 QDs exhibit a superior activity with an overpotential at a current density of 10 mA cm-2 is 262 mV, which is lower than those of Ni-Fe LDHs (285


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mV) and CoFe2O4 (333 mV) in this work. It should be noted that although AT NiFe2O4 QDs exhibit an overpotential at 10 mA cm-2 higher than that of RuO2 (255 mV in Figure 3b), the AT NiFe2O4 QDs show an overpotential of 305 mV (lower than the 311 mV for RuO2) when the current density increased to 40 mA cm-2 versus RHE, clearly indicating a higher catalytic activity for AT NiFe2O4 QDs (see SI for detailed description). Notably, the OER catalytic performance of AT NiFe2O4 QDs is better than most of the nonprecious metal oxide catalysts in basic media, particularly compared with previously reported NiFe2O4-based catalysts (Table S3). Additionally, the LSV curves of these samples, with the current density based on the mass activity, are shown in Figure S2 and further confirm that the AT NiFe2O4 QDs have the highest electrocatalytic activity and cost effectiveness for the OER compared with the other samples investigated. Moreover, to further elucidate the OER kinetics, the corresponding Tafel plots were obtained at a scan rate of 10 mV s-1 and were obtained for all the abovementioned catalysts. Of note, a smaller Tafel slope leads to an increased OER rate with an increase in overpotential. As shown in Figure 3c, Tafel slopes of 37, 93, 86, 47, and 53 mV dec-1 correspond to AT NiFe2O4 QDs, AT NiFe2O4 QDs/PS, Ni-Fe LDHs, CoFe2O4, and RuO2, respectively. The as-synthesized AT NiFe2O4 QDs exhibit the smallest Tafel slopes, suggesting that the favourable intrinsic OER kinetics of this unique AT QD structure is comparable to the RuO2 because of the improved electron transfer capacity and the synergetic effect between an atomically thin thickness and a small lateral size. Additionally, AT NiFe2O4 QDs show a nearly 100% Faradaic yield for the OER with a stable O2 evolution rate of 19.8 µmol h-1 (Figure 3d), indicating that the nature of AT NiFe2O4 QDs catalysing the oxidation current can be ascribed to the oxygen production process.52


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Figure 4. (a) CP and CA plots of AT NiFe2O4 QDs recorded for over 100 h in 1 M KOH. (b) LSVs of AT NiFe2O4 QDs before and after 5000 cycles. (c) The Multi-potential process of AT NiFe2O4 QDs. The potential was started at 1.49 V and ended at 1.58 V, with an increment of 10 mV per hour. (d) A digital image showing the chemical stability of AT NiFe2O4 QDs in 1 M KOH for 120 h. (E) An image of a colloidal suspension of AT NiFe2O4 QDs dispersed in 1 M KOH for 20 days. Considering that the electrochemical stability of a catalyst is very important for the OER and directly affects the cost of industrial production (hydrogen and oxygen),32,60 the long-time chronoamperometry (CA) and chronopotentiometry (CP) curves of AT NiFe2O4 QDs for OER were measured to estimate the lifetime in 1.0 M KOH. Both curves shown in Figure 4a illustrate


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that the AT NiFe2O4 QDs remain stable for at least 100 h. The outstanding long-term stability can be ascribed to the synergetic effects of a high resistance to chemical attack and an intrinsic chemical stability. To further evaluate the stability of AT NiFe2O4 QDs, as shown in Figure 4b, after 1000 and 2000 continuous cycles, AT NiFe2O4 QDs present analogous i–V curves with negligible loss in the anodic current and agree with the first cycle, further confirming the outstanding long-term stability of AT NiFe2O4 QDs. Figure 4c displays the multi-potential step curve of AT NiFe2O4 QDs in 1.0 M KOH with the potential increasing from 1.49 to 1.58 V (10 mV per hour). The current density immediately levels off at 14 mA cm-2 at the initial potential value and remains constant for the next 1 h; the other steps also show similar results, confirming that the as-synthesized AT NiFe2O4 QDs possess a robust electrochemical stability. Inspired by previous studies, the long-term electrocatalytic stability for water splitting cannot be represented by the overall stability of AT NiFe2O4 QDs completely, since partial corrosion can take place.52,61 In particular, AT NiFe2O4 QDs are likely to yield partial corrosion because of the relatively high contact surface area between AT NiFe2O4 QDs and the basic electrolyte of 1.0 M KOH. With the aim of evaluating the total stability, the crystalline structure of AT NiFe2O4 QDs was examined by an HRTEM measurement. As shown in Figure 4d, the HRTEM images of samples obtained after the stability testing reveal that AT NiFe2O4 QDs can maintain their crystalline structure after long-term tests. Moreover, the monolayer nanosheets can aggregate easily, thus, it is important to examine the stability of dispersions after a long-term dispersion in a basic medium. As shown in Figure 4e, after being dispersed in 1 M KOH for 20 days, the clear Tyndall effect of a stable colloidal suspension of AT NiFe2O4 QDs was observed, revealing that the AT NiFe2O4 QD electrocatalysts can be well dispersed in a 1 M KOH electrolyte for a long term. ICP-OES tests of the KOH electrolyte after periods of 5, 10, and 20 days (Table S4)


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indicate relatively low Ni and Fe amounts in the electrolyte, further confirming the remarkable chemical stability of AT NiFe2O4 QDs. Based on the robust stability and the combination of high specific and mass activities in basic media, we can conclude that AT NiFe2O4 QDs are an efficient and versatile electrocatalyst for the OER. It is well known that electrocatalytic activity depends on the inherent features of a material (i.e., chemical composition, crystal structure, and surface features (material thicknes, lateral size, and defect sites in this study)), which can strongly affect or tailor its intrinsic properties and its electrocatalytic performance.62 To obtain further information about the high performance of AT NiFe2O4 QDs for the OER, the interaction among these inherent features and catalytic performance is worth noting. A series of relevant samples were prepared by tuning the diameters of the PS template, which was considered to be one of the crucial factors influencing the formation of NiFe2O4 nanosheets, and the corresponding catalytic performance for the OER was then examined. Two different PS templates 1000 and 2000 nm in diameter were synthesized, while the other parameters were kept constant and are denoted as S-1000 and S-2000, respectively. For comparison, bulk NiFe2O4, the most widely existing form of spinel-structured NiFe2O4 species, was also tested under the same conditions and is denoted as S-bulk. It is noteworthy that the AT NiFe2O4 QDs discussed above grow on PS with a 300 nm diameter and are denoted as S-300 in the discussed below. The morphologies of S-300, S-1000, S-2000, and S-bulk were determined using TEM. Representative TEM images (Figures 2 and 5) for S-300, S-1000, S-2000, and S-bulk were chosen to investigate the micro-architecture of each sample. The images show that the lateral size of the nanosheets is extended when the PS diameter is increased to 1000 nm; we can clearly observe that the lateral size of S-1000 is approximately 20 nm, and the thickness is


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Figure 5. (a, b) TEM and (c) HRTEM images of S-1000. (d, e) TEM and (f) HRTEM images of S-2000. approximately 0.8 nm, which is in good agreement with S-300 (Figure S4). A larger lateral size (~1000 nm) of nanosheets was observed with a PS diameter of 2000 nm, and it should be mentioned that the nanosheets ~1000 nm in lateral size and ~0.8 nm in thickness (Figure S4) cannot adhere to the template directly. It is interesting that the lateral size of the NiFe2O4 nanosheets decreases with a reduction in PS diameter and that it is mainly due to the difference in the density of surface hydrophilic groups on PS of different diameters (see SI for detailed description). To directly identify the crystalline structures for three samples, FFT patterns were obtained from the HRTEM images, and the diffraction patterns were identified. The highly uniform arrangement of nanodomains in S-300, S-1000, and S-2000 is shown by the identical diffraction patterns in their FFT images. Moreover, the AFM and XRD images (Figures S4 and S5) indicate that the nanosheets grown on different PS diameters possess the same thickness and


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Figure 6. (a) The Fe 2p spectra and (b) O 1s spectra of S-300, S-1000, S-2000, and S-bulk. (c) LSV curves and (d) corresponding Tafel plots of S-300, S-1000, S-2000, and S-bulk for the OER in 1 M KOH. species. Notably, the corresponding XRD peaks of S-300, S-1000, and S-2000 are indexed to the cubic NiFe2O4 species, with the peaks at 30.3, 35.7, and 43.4° all corresponding to JCPDS no. 10-0325. By contrast, the pattern of S-bulk exhibits two more peaks at 57.4 and 62.9°, revealing that there are nearly no additional crystal orientations for S-300, S-1000, and S-2000, except for [400], because of their one unit-cell thickness. In addition, ICP-OES measurements (Table S1)


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suggest that the Ni : Fe mole ratios of S-300, S-1000, and S-2000 all correspond to the composition of NiFe2O4, confirming that the three samples have identical chemical compositions. Consistent with the XRD and ICP-OES results, the XPS peaks in the Fe 2p and O 1s regions of S-300, S-1000, and S-2000 (Figure 6a and b) possess identical binding energies, revealing the same chemical composition for these samples. Notably, the relative peak intensity of O2 in comparison with O1 in the O 1s region exhibits a significant increase going from S2000 to S-300, indicating that the atomically thin NiFe2O4 nanosheets with a smaller lateral size possess more oxygen vacancies.17,59 This could be attributed to a greater increase in both edge length and low-coordinated oxygen atoms with decreasing lateral size.44 It is evident that the low-coordinated oxygen atoms in NiFe2O4 are more easily removed under an inert atmosphere during the calcination process to form oxygen vacancies since O2- atoms with fewer Fe-O bonds are more easily cleaved through the breaking of the Fe-O bond (see Figure S6 and SI for detailed description). For this reason, a reduction of the lateral size of NiFe2O4 nanosheets can increase the number of low-coordinated oxygen atoms; therefore, there is likely to be significantly more oxygen vacancies. As shown in Figure S6, the NiFe2O4 nanosheets possess high-coordinated surface O2- atoms and low-coordinated edge O2- atoms. During the synthesis of S-300 (i. e. AT NiFe2O4 QDs), the presence of a longer edge length could increase the percentage of the edge O2- atoms with a relatively low coordination number of 2 compared with the surface O2- atoms with a coordination number of 4. That means the S-300 with a longer edge length possesses more oxygen vacancies on their edges, which can significantly reduce the coordination number of the neighbouring Fe3+ atoms and can be employed as active sites for water oxidation. The successful synthesis of these samples, rich or poor in oxygen vacancies, provides a broader sense of perspective to determine the reason for the high OER performance of S-300. For now, all the


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inherent features of these three samples (i.e., thickness, composition, and crystalline structure), except for lateral size and the corresponding density of oxygen vacancies, were verified to be nearly identical in this experiment. Thereby, with the aim to better understand the interplay between lateral size and catalytic performance, the intrinsic catalytic ability of S-300, S-1000, and S-2000 was examined by an elaborate analysis for comparison. LSV curves of four different samples provide a quantitative analysis of the OER performance of each size of atomically thin NiFe2O4 nanosheets, and the result is shown in Figure 6c. With an overpotential value of 380 mV for S-bulk at a current density of 10 mA cm-2, the S-300, S-1000, and S-2000 samples show a clear decrease in overpotential with decreasing lateral size. Notably, the S-300 with the smallest lateral size shows the lowest overpotential value and correspondingly shows the highest catalytic activity because of its high density of active sites with a low coordination number, which is consistent with previous reports.17,32,46 The OER kinetics of these samples were further measured by the corresponding Tafel plots. Tafel slopes of approximately 40, 47, and 79 mV dec-1 for S1000, S-2000 and S-bulk, respectively, are shown in Figure 6d. Interestingly, the Tafel slopes for S-300, S-1000 and S-2000 decrease with the lateral size of the NiFe2O4 nanosheets, suggesting the low-coordinated atoms on the surface facilitate the intrinsic OER kinetics. The assynthesized S-300 exhibits the smallest Tafel slope, making it the most efficient electrocatalyst among the adopted samples and demonstrating the enhancement effect of small lateral size on the OER activity. It is of note that the greatly improved electrocatalytic oxygen evolution properties of AT NiFe2O4 QDs can be ascribed to the synergetic effects of small lateral size and corresponding high density of surface oxygen vacancies. Especially, the oxygen vacancy has been well verified to be the catalytic active site for OER because it can overcome the hurdle of the adsorption of


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H2O; specifically speaking, the adsorption of H2O can more easily occur at the neighbouring low-coordinated Fe3+ site.17 Moreover, the existence of surface oxygen vacancies in an electrocatalyst can also lead to a complete delocalization of the neighbouring electrons that previously occupied the O 2p orbital.17,38 That means the different oxygen vacancy concentrations can reflect the relative positions of transition metal 3d bands to oxygen 2p bands in crystalline structure, as well as the corresponding catalytic activity.63,64 Herein, to further systematically investigate the influence of oxygen vacancy concentrations on the OER activity of catalysts, a series of AT NiFe2O4 QDs samples with different oxygen vacancy concentrations were prepared (using the AT NiFe2O4 QDs grown on 300 nm PS particles as the basic sample) and measured by XPS (Figure S7) and ICP-OES (Table S5). Of note, the calcining treatment duration has a direct influence on the concentration of oxygen vacancies for AT NiFe2O4 QDs in this experiment. By calcining treatment for 3, 2, and 1 h with the other parameters kept constant (see SI for detailed description), respectively, the resulting samples are denoted as S-3, S-2, and S-1, respectively. The original AT NiFe2O4 QDs by calcining treatment for 6 h is denoted as S-6. Based on the calculated Ni2+/Ni3+ and Fe2+/Fe3+ ratios from XPS spectra and ICP-OES results (Table S5), the actual molecular formulas of S-6, S-3, S-2, and S-1 are Ni1.02Fe2O3.81, Ni1.02Fe2O3.97, Ni1.02Fe2O3.99, and Ni1.02Fe2O4.01, respectively. Combined the above results and the analysis of O 1s spectra, we can clearly observe an obvious trend in oxygen vacancy concentrations with a decrease in the order S-6 > S-3 > S-2 > S-1. Judging from LSV curves (Figure S8a), these samples display different electrocatalytic activities for OER, and the S-6 shows the best performance with a large current density of 60 mA cm-2 at a overpotential of 327 mV, about 2.2, 4.4, and 9.3 times of S-3, S-2, and S-1, respectively, suggesting the catalysts rich in oxygen vacancies have an improved catalytic activity for OER. Moreover, the S-6 exhibits the


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smallest Tafel slopes (Figure S8b), suggesting its favorable intrinsic OER kinetics. Referred to previous reports, this is associated with the generally proposed adsorbates evolution mechanism that relates to the increase in redox activity of the B-site cations in bimetallic oxides.63-65 Illuminated by the research results of the formation of Fe4+ involved in the catalytic process of NiFe oxides,66 the catalytic cycle of AT NiFe2O4 QDs with oxygen vacancies in this work were performed (Figure S9). As described in Step 1, the surface oxygen vacancies of AT NiFe2O4 QDs are electrochemically filled with OH(aq), which leads to a surface stoichiometry close to that of AB2O4. The catalytic cycle takes place on coordinated hydroxyl groups similar to the ones depicted for various homogeneous catalytic cycles. Although the formation of the O-O bond (Step 3) and the proton extraction of oxyhydroxide group (Step 4) are the rate determining steps, the catalytic cycle starts at Step 1. This means that the more presence of oxygen vacancies on the surface of the catalysts will lead to the more occurrences of catalytic cycles. This is confirmed by the much higher Aechem of S-6 than that of other samples (Figure S8c). Besides, the oxygen vacancy can induce the surrounding delocalized electrons to be excited to the conduction band, which can improve the electron transfer in spinel structured oxides.17 The enhanced conductivity of S-6 is supported by EIS measurements (Figure S8d). Thus, the catalysts rich in oxygen vacancies would exhibit a superior performance for the OER. Clearly, the H2O molecules are adsorbed on the catalyst surface during the initial stage of water oxidation. According to Alexis T. Bell’s research,65 the mechanism described in Figure S9 can be modified as following reactions: 2H2O + * → OH* + H2O + e- + H+

(1)

OH* + H2O → O* + H2O + e- + H+

(2)

O* + H2O → OOH* + e- + H+

(3)


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OOH* → O2 + e- + H+

(4)

In this regard, the adsorption energy of H2O molecules on the edge oxygen vacancies (that is, the neighbouring Fe3+ atoms with a reduced coordination number) is vital for improving the OER activity of AT NiFe2O4 QDs.17 To get insights into the role of edge active sites, DFT calculations were performed for the OER process. Taking NiFe2O4 as an example, which had been traditionally regarded as a non-ideal candidate for an OER catalyst, we present a new AT QD structural motif in this study. As show in Figure 7, DFT calculations reveal that the adsorption energy (absolute value) of 1-coordinated Fe3+ atoms is 0.71 eV, higher than that for 2-coordinated Fe3+ atoms (0.66 eV), indicating that the 1-coordinated Fe3+ is more favourable for adsorbing H2O molecules. The calculated adsorption energy of 5-coordinated surface O2- atoms is 0.39 eV, much higher than that for 6coordinated O2- atoms on the surface. From the above results, we can conclude that a lower value of the coordination number (CN) can result in an outstanding catalytic activity. Notably, the adsorption energy of H2O molecules on Fe3+ atoms gradually increases as the corresponding coordination number decreases from 6 to 1, further demonstrating that 1- or 2-coordinated Fe3+ atoms display the highest reactivity for water oxidation. In conclusion, AT NiFe2O4 QDs with the smallest lateral size possess increased edge length, which provides an increased number of low-coordinated edge oxygen atoms that one would expect can be removed more easily to the high-coordinated surface oxygen atoms during the calcination process. As a result, the number of 1- or 2-coordinated Fe3+ atoms with a relatively high catalytic activity existing on the nanosheets edge increases, and accordingly, the AT NiFe2O4 QDs show the highest catalytic activity for the OER.


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Figure 7. (a) Crystal structure of AT NiFe2O4 QDs with surface-exposed (400) facets. (b) A depiction of 3, 4, and 6 coordinated Fe3+ atoms. The O2- atoms binding with Fe3+ atoms can be removed from the NiFe2O4 nanosheets to form oxygen vacancies (that is, the low-coordinated Fe3+ atoms). (c) The calculated adsorption energy of a H2O molecule on different coordinated Fe3+ atoms.


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In addition to the intrinsic reactivity, the number of active sites is also essential to achieve an excellent electrocatalytic activity for the OER. Notably, the as-synthesized AT NiFe2O4 QDs have the smallest lateral size, which can clearly increase the number of highly exposed active edge sites, thus, affording the largest Aechem.56,67-69 To further confirm this, we compared the Aechem values of S-300, S-1000, S-2000, and S-bulk. As shown in Figure 8a, the double-layer capacitance (Cdl) of S-300 is confirmed to be 15.1 mF cm-2 based on the slope calculations, which is remarkably higher than those of S-1000 and S-2000 (12.8 and 10.5 mF cm-2, respectively) and even larger than that of S-bulk (5.2 mF cm-2). Thus, compared with the other atomically thin counterparts, S-300 exposes a high density of active edge sites to the electrolyte. Additionally, the semicircular diameter in the electrochemical impedance spectroscopy (EIS) plot of atomically thin S-300 is substantially smaller than those of bulk NiFe2O4 and the other counterparts (Figure 8b and Table S6) because of the fast electron transfer and small contact impedance originating from the effects between a prominent edge and quantum confinement, as determined by its tailor-made advanced surface structure. That is, the AT QD structure can longitudinally increase the number of exposed low-coordinated atoms that pre-existed inside the 2D NiFe2O4 nanosheets, making it possible to obtain an increased number of active edge sites, while the crystalline structure and crystallinity of the nanosheets were maintained.44 This structure can produce an additional vertical mass-transfer pathway to the nanosheet plane, which favours electron transport through these long edges, whereas the common 2D NiFe2O4 nanosheets only possess a parallel pathway to their surface. This feature of the AT QD structure can greatly accelerate ion transport throughout the different layers of NiFe2O4 in the entire catalytic film of the electrode, which facilitates the access of ions to the entire surface area.43 Thus, we can conclude that the high activity of AT NiFe2O4 QDs not only


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Figure 8. (a) Plots used for evaluating the Cdl of S-300, S-1000, S-2000, and S-bulk. (b) Nyquist plots of S-300, S-1000, S-2000, and S-bulk. results from an increased fraction of exposed low-coordinated edge atoms but also an enhanced electrical conductivity. AT NiFe2O4 QDs exhibit an excellent catalytic performance arising from the following facts: (i) An intrinsically low charge transfer resistance enhances the charge transport between the electrocatalytic phase and the electrode, facilitating the accessibility of active sites to the charge offered by the electrode. (ii) The AT QD structure being ~8 nm in lateral size can not only provide more active edge sites (oxygen vacancies) but also maximize the ratio of active surface sites to the total number of atoms. (iii) Abundant oxygen vacancies on the edge of nanosheets can reduce the coordination number of neighbouring Fe3+ atoms, thus resulting in an improved catalytic reactivity. 4. CONCLUSION In conclusion, the controllable growth of AT NiFe2O4 QDs ~0.8 nm in thickness and ~8 nm in lateral size was successfully achieved by a mild ultrasonic treatment. The as-synthesized material


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exhibits significant improvements in OER electrocatalytic activity and excellent durability. We highlighted that the enhanced catalytic activity of AT NiFe2O4 QDs is mainly due to an improved reactivity and to increasing the number of exposed active edge sites arising from the AT QD surface structure. We also demonstrated that there is a negative correlation between the lateral size and OER performance of NiFe2O4 nanosheets. A reduction in the lateral size can increase the number of low-coordinated oxygen atoms, and this leads to significantly more oxygen vacancies that can lower the adsorption energy of H2O, increasing the AT NiFe2O4 QD catalytic OER efficiency. The controllable and mild strategy to obtain the high-performance AT NiFe2O4 QDs reported here not only offers an effective way to design and synthesize OER electrocatalysts based on spinel-structured oxides as potential substitutes for non-abundant noble metal catalysts but also opens up a promising approach to promote the development of other electrocatalysts based on transition-metal oxides for the field of renewable energy systems and energy-intensive industries. ASSOCIATED CONTENT Supporting Information Supporting information contained detail for the estimation of electrochemically active surface area, electrochemical impedance spectroscopy (EIS) analysis, Faradaic efficiency measurements, and DFT calculations. AFM images, XRD spectra and electrocatalytic mass activity of samples can be found in supporting information. The ICP-OES measured value of Ni : Fe mole ratio and the geometric values of series resistance (Rs) and charge transfer resistance (Rct) for samples, comparison of the electrocatalytic performance of AT NiFe2O4 QDs versus OER electrocatalysts reported recently can also found in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.


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ACKNOWLEDGMENT This research was supported by the National Science Foundation of China (No. 21345003), the Fundamental Research Funds for the Central Universities (Grant no. lzujbky-2016-k08), the Natural Science Foundation of Gansu (145RJZA132), the Key Laboratory of Catalytic Engineering of Gansu Province and Resources Utilization, Gansu Province for ﬁnancial support. REFERENCES (1) Walter, M.; Warren, E.; McKone, J.; Boettcher, S.; Mi, Q.; Santori E.; Lewis, N. Chem. Rev. 2010, 110, 6446–6473. (2) Mallouk, T. E. Nature Chem. 2013, 5, 362–363. (3) Debe, M. Nature 2012, 486, 43–51. (4) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. Science 2011, 334, 1383–1385. (5) Lv, Z.; Mahmood, N.; Tahir, M.; Pan, L.; Zhang, X.; Zou, J. Nanoscale 2016, 8, 18250–18269. (6) Feng, L.; Yu, G.; Wu, Y.; Li, G.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X.; J. Am. Chem. Soc. 2015, 137, 14023–14026. (7) Zhong, X.; Jiang, Y.; Chen, X.; Wang, L.; Zhuang, G.; Li, X.; Wang, J. J. Mater. Chem. A 2016, 4, 10575–10584. (8) Shalom, M.; Ressnig, D.; Yang, X.; Clavel, G.; Fellinger, T.; Antonietti, M. J. Mater. Chem. A 2015, 3, 8171–8177.


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Lateral-Size-Mediated Efficient Oxygen Evolution Reaction: Insights

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