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Generalizable, Electroless, Template-Assisted Synthesis and Electrocatalytic Mechanistic Understanding of Perovskite LaNiO3 Nanorods as Viable, Supportless Oxygen Evolution Reaction Catalysts in Alkaline Media Coray L. McBean,† Haiqing Liu,† Megan E. Scofield,† Luyao Li,† Lei Wang,† Ashley Bernstein,† and Stanislaus S. Wong*,†,‡ †

Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, United States Condensed Matter Physics and Materials Sciences Division, Brookhaven National Laboratory, Building 480, Upton, New York 11973, United States

‡

S Supporting Information *

ABSTRACT: The oxygen evolution reaction (OER) is a key reaction for water electrolysis cells and air-powered battery applications. However, conventional metal oxide catalysts, used for high-performing OER, tend to incorporate comparatively expensive and less abundant precious metals such as Ru and Ir, and, moreover, suffer from poor stability. To attempt to mitigate for all of these issues, we have prepared one-dimensional (1D) OER-active perovskite nanorods using a unique, simple, generalizable, and robust method. Significantly, our work demonstrates the feasibility of a novel electroless, seedless, surfactant-free, wet solution-based protocol for fabricating “high aspect ratio” LaNiO3 and LaMnO3 nanostructures. As the main focus of our demonstration of principle, we prepared as-synthesized LaNiO3 rods and correlated the various temperatures at which these materials were annealed with their resulting OER performance. We observed generally better OER performance for samples prepared with lower annealing temperatures. Specifically, when annealed at 600 °C, in the absence of a conventional conductive carbon support, our as-synthesized LaNiO3 rods not only evinced (i) a reasonable level of activity toward OER but also displayed (ii) an improved stability, as demonstrated by chronoamperometric measurements, especially when compared with a control sample of commercially available (and more expensive) RuO2. KEYWORDS: catalysis, oxygen evolution reaction, template synthesis, perovskite, electrochemistry

1. INTRODUCTION Perovskite metal oxides are characterized by intriguing dielectric, piezoelectric, electrostrictive, and electro-optic properties. As such, recent emphasis has been placed on their potential applicability as components of transducers, capacitors, actuators, high-k dielectrics, dynamic random access memory devices, field-effect transistors, and logic circuitry.1−6 More recently, metal oxide and perovskite materials have been investigated as viable alternatives to expensive noble metals, such as Pt, Ru, and Ir, for use within fuel cells and water electrolysis cells as active oxygen evolution and oxygen reduction reaction catalysts. In particular, the oxygen evolution reaction (OER) denotes a versatile reaction that lies at the core of not only successful water splitting but also effective rechargeable Li−air and Zn−air batteries. This reaction, characterized by the transfer of 4 electrons, proceeds via eq 1, as follows:7 2H 2O → 4e− + 4H+ + O2 (E° = − 1.23 V) © XXXX American Chemical Society

One of the best metal oxide catalysts reported to date for OER consists of RuO2, although it possesses a large overpotential of 0.3 eV and gives rise to a relatively poor stability. 7 Unfortunately, as a material, because of its incorporation of Ru itself, RuO2 is also inherently more expensive than more traditional and more plentiful transition metals. Therefore, there is a significant need (i) to lower the measured overpotential, (ii) to increase long-term stability, as well as (iii) to find cheaper, more abundant alternatives to conventional binary metal oxide systems, such as RuO2 and IrO2. As an example, cobalt-doped LaMnO3/nitrogen-doped carbon nanotubes have been found to demonstrate a reasonably high activity of 27 mA cm−2 at 0.9 V vs SCE for OER.8 Moreover, both in situ-generated cobalt−manganese oxide− CNT/heterostructures9 as well as a family of CoO-Ni-NiCo/NReceived: May 15, 2017 Accepted: July 4, 2017

(1) A

DOI: 10.1021/acsami.7b06855 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces doped CNT heterostructures10 are reported to have significantly out-performed commercial IrO2 particles for OER under alkaline conditions. With respect to OER, the specific perovskite we have focused on, namely, LaNiO3, has also shown enhanced activities for the OER, comparable in magnitude to that associated with RuO2 and IrO2.11 Specifically, the 3+ oxidation state of Ni, associated with the catalytically active site within LaNiO3, nominally allows for an eg orbital occupancy of one electron within the octahedral Ni cation, although it has been suggested that the ideal theoretical occupancy of this orbital is actually ∼1.2 electrons.12 In principle, the average occupancy level of the eg orbital for Ni within LaNiO3 can be increased toward the more desirable 1.2 electrons through the introduction of oxygen vacancies. This process is necessarily accompanied by the corresponding conversion of Ni3+ to Ni2+, so as to maintain the overall charge balance and charge neutrality within the material. Moreover, LaNiO3 represents a suitably useful model system, because it possesses very good electrical conductivity, particularly for a metal oxide,13,14 a trait that is fundamentally conducive toward enhancing overall catalytic activity. This implication therefore is that with this metal oxide alone, we should be able to completely remove any form of an underlying conductive carbon substrate, which is commonly and ubiquitously employed in OER catalysis, thereby potentially simplifying the overall electrochemical setup. Hence, as a motivation for this study, we believe that we can potentially impact the observed OER catalytic activity by controlling and tailoring the degree of oxygen vacancy, structure, and surface area of our as-prepared LaNiO3 samples. Specifically, our objective in this Article has been to synthesize “high aspect ratio” morphologies of LaNiO3, because these motifs are anisotropic and should provide for long and electrically continuous segments for the electrons to traverse. It is our expectation that these nanorod motifs can not only minimize the number of interparticle boundaries, which can adversely hinder conductivity, but also maximize the exposed surface area (relative to the bulk) that can partake in actively catalyzing OER activity. Hence, our morphological manifestation of LaNiO3 denotes a favorable design for creating wellperforming catalysts possessing reproducibly enhanced activities for OER. This is indeed what we have observed herein. In terms of prior studies for this material, a number of synthetic techniques have been previously employed for the generation of LaNiO3 including electrospinning,15,16 several forms of epitaxial growth,17−20 molten salt methods,21 hightemperature solid-state reactions, hydrothermally assisted annealing of coprecipitants,22 as well as sol−gel methods.23,24 With respect to morphological distinctiveness, discrete and well-defined individual submicrometer cubes have been synthesized using a hydrothermally assisted annealing of coprecipitants. Moreover, a number of these as-prepared structures are not fully monodisperse in terms of either size or shape, denoting key structural parameters that are often relevant, important, and necessary prerequisites for the proper control and understanding of the catalytic process. Our template-assisted synthesis attempts to address all of these issues. We focus on the creation of a “high aspect ratio” motif, namely, the formation of discrete, pure, and crystalline rods possessing an average diameter of ∼125 nm. Moreover, we have been able to produce nanowires with diameters as low as 99 nm, which, to the best of our knowledge, represents one of the smallest diameters ever produced for the fabrication of

morphologically well-defined LaNiO3. In general, our crystalline one-dimensional (1D) nanostructures possess (a) high aspect ratios, (b) fewer lattice boundaries, (c) exposed crystalline planes, created in the absence of potential catalytic inhibitors such as either surfactants or capping agents, and (d) a tunable number of oxygen vacancy defect sites, all of which denote desirable features for high-performance electrochemical catalysts.25−33 In addition, our “high aspect ratio” nanostructures are highly porous with a roughened surface, and these attributes therefore can potentially increase the amount of active exposed surface area available for possible reaction. A significant advantage of our synthetic scheme is the avoidance of extended annealing treatments (i.e., in excess of 10 min), which would tend to promote sintering. Sintering may adversely result in the formation of smoother surfaces, pore population depletion, and, therefore, an overall reduction in the catalytically active surface area available. Indeed, the templatemediated synthetic protocol described herein facilitates the generation of LaNiO3 nanorods with minimal annealing times. It should be noted that annealing in air for a short time duration can allow for easy, single-step control over the extent of oxygen vacancies within the LaNiO3 sample by simply varying the annealing temperatures used. More importantly, to the best of our knowledge, we are the first to create crystalline, chemically pure, and morphologically well-defined LaNiO3 nanorods using an electroless, surfactant-free, and seedless reaction protocol. Moreover, we have strengthened our discussion with a detailed reaction scheme and accompanying discussion that outlines and explains the three discrete reaction stages involved, in significant detail. Importantly, because of the inherently “modular” nature of perovskites, by simply tailoring the identity of the precursor metal species, the synthesis protocol employed in this study represents a flexible, straightforward, and robust strategy for potentially producing a wide range of perovskite nanomaterials characterized by various diameters and lengths. In this Article, we focus on achieving the production of monodisperse LaNiO3 nanorods in the absence of either complexing agents,34 hydrothermal reaction steps, a combination of lengthy drying22 and high-temperature annealing treatments,35 or even the use of singular reaction environments, such as O2-rich atmospheres,36 which have tended to characterize previous synthetic forays in this area. Hence, a significant aspect of the novelty of what we present is that we have been able to adapt, for the first time, the recognized and established template methodology to the synthesis of pure “high aspect ratio” motifs of LaNiO3. Given that there are relatively few reports of the synthesis of “high aspect ratio” perovskites to begin with, our advance represents an important result, in and of itself. Furthermore, our protocol has a broad and universal applicability in terms of our potential to fabricate other analogous “high aspect ratio” perovskite materials that have yet to be generated by any other means. As such, we have confirmed the versatility and generalizability of our methodology through the concomitant production of LaMnO3. By comparison with LaNiO3, LaMnO3 in addition to LaMnO3-based systems, such as but not limited to Co-doped LaMnO3 and Ca-substituted LaMnO3, demonstrate measurable electrochemical activity for OER and the oxygen reduction reaction (ORR).8,11,37 However, by contrast with LaNiO3, LaMnO3 is a poor electrical conductor but remains a promising bifunctional catalyst, due in part to its relatively minimal B


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ACS Applied Materials & Interfaces environmental impact coupled with the reasonably low cost of its constituent metal atoms. Significantly, its ORR activity is due in large part to the ability of this material to maintain a cationic deficiency and to thereby facilitate a considerable degree of oxygen mobility.38 Moreover, as these oxides possess the ability to undergo both OER and ORR, prior groups have found that LaMnO3 and corresponding LaMnO3 derivatives can be used as components of water electrolyzers, fuel cells, and metal−air batteries.39 To summarize, we have developed a novel and reasonably simple protocol for the generation of LaNiO3 possessing a nanorod morphology, which should be highly beneficial for OER. The combination of the anisotropic 1D nature of our nanorods, coupled with their high surface area, pure chemical composition, as well as monodisperse size and shape, offers a number of unique advantages for catalysis. In fact, we have found that our facilely produced nanorods show not only a reasonable OER activity but also an overall better stability, especially as compared with commercially available RuO2. We emphasize that all samples were tested under similar conditions within a desirable alkaline environment. Moreover, we correlated the observation of generally improved OER performance with the lower annealing temperatures of our asprepared LaNiO3. To reiterate this point, a significant advantage of our OER-active LaNiO3 samples was that we were able to eliminate the need for either dopants or an additional conductive carbon additive, the latter of which has been commonly used to specifically enhance OER performance for not only previously reported LaNiO3 systems but also other conventional binary metal oxides, such as Co3O4.40

Table 1. List of Relevant Reaction Equations for the Multistep Production of LaNiO3 eq no. 2 3 4 5 6

chemical reaction involved LaCl3(aq) + NiCl2(aq) + 5NaOH(aq) → La(OH)3(s) + Ni(OH)2(s) + 5NaCl(aq) 2Ni(OH)2 + NaClO → 2NiOOH + H2O + NaCl 2NiOOH → Ni2O3 + H2O 2La(OH)3 → La2O3 + 3H2O La2O3 + Ni2O3 → 2LaNiO3

with the basic solution. The template membrane was allowed to remain in the basic solution for 30 s, prior to removal from the base, followed by rinsing with deionized (DI) water along with a mild sonication step, which detached much of the loose bulk precipitate localized on the template surface. We note that we could easily reuse stock solutions of LaCl3(aq), NiCl2(aq), and NaOH to create sizable quantities of product. Specifically, the 2 mL stock solution containing LaCl3(aq) and NiCl2(aq) precursors (maintaining respective concentrations of 0.9 M), into which the templates are initially immersed, remains continuously recyclable (without the need for replenishment). As such, we can initiate multiple runs, involving several sets of templates, with one stock solution to ultimately generate LaNiO3, characterizing an overall process that ended up depleting the initial La 3+ and Ni 2+ concentrations within the initial solution to a negligible concentration. Hence, this protocol suggests that we can efficiently convert LaCl3 and NiCl2 to LaNiO3. Moreover, the basic NaOH solution can also be reused, because the initial stock OH− concentration is in vast excess of what is needed to precipitate both the La and the Ni hydroxides within the spatial confines of the template pore channels. Once the templates were oven-dried at 80 °C for 30 min, both sides of each template membranes were subsequently polished with strips of 2000 grit sandpaper to further remove any remaining bulk metal hydroxide. The templates were then sonicated and rinsed with DI water either 2−3 times or until there were no visibly dispersed particles in solution, apparent after sonication. Additional drying was performed at 60 °C either for 30 min or until the product had completely dried. Stage 2: Conversion of La(OH)3−Ni(OH)2 into La(OH)3−NiOOH Composite Nanorods. The conversion reaction into La(OH)3− NiOOH composite rods (subsequently referred to as the “LNOIntermediate”) occurred through the following steps. The polycarbonate template, collected after drying, was dissolved by washing in dichloromethane five separate times, followed by once in H2O, leaving the rods to freely disperse. The as-obtained powder was then dried in an ambient atmosphere. The resulting dried “white-to-pale green” powder was subsequently left to soak within a 2.5 M NaOH/1.25% NaClO solution for 1.5 h, with the first 10 min under mild sonication. The dispersion darkened in hue, as Ni(OH)2 was converted into NiOOH, as described by eq 3 in Table 1.41 Upon centrifugation of the dispersion, a brown-black powder was isolated and washed with deionized (DI) water four times to remove excess NaOH and NaClO. Ultimately, the powder was air-dried at 80 °C. Stage 3: Converting La(OH)3−NiOOH Composite Nanorods to LaNiO3 Nanorods. Annealing in air was performed at either 600, 700, 800, or 900 °C, denoting samples subsequently labeled as LNO-600, LNO-700, LNO-800, and LNO-900, respectively, using a preheated tube furnace run for 10 min to obtain a black LaNiO3 powder in a process described by eqs 4−6 in Table 1.42,43 After annealing, the contents of the ceramic boat were quenched by immediately removing the boat from the furnace and allowing it to revert to room temperature by natural air cooling. Finally, the as-obtained black powder was washed three more times in DI H2O to remove any residual NaCl that may have formed from the degraded residual NaClO. The as-obtained, isolated black powder was ultimately ovendried in air. Parallel “Bulk” Synthesis. A solution of LaCl3(aq) and NiCl2(aq) was slowly added to a NaOH(aq) solution, resulting in the immediate

2. EXPERIMENTAL SECTION 2.1. Synthesis. As reagents for our reactions, we used manganese(II) nitrate tetrahydrate (Alfa Aesar, 98%), hydrated lanthanum(III) chloride (Acros Fisher, 64.5−70% LaCl3·xH2O), nickel(II) chloride hexahydrate (Fisher Scientific), concentrated hydrochloric acid (Fisher Chemicals, NF/FCC Grade), sodium hydroxide (EMD, ≥99%), sodium hypochlorite (Acros Fisher, 5%), dichloromethane (DCM, Acros Fisher, ≥99%), 200 nm track-etched polycarbonate membranes (Whatman), and 3 M (Imperial Wet or Dry) sandpaper. With the exception of NiCl2·6H2O and LaCl3·xH2O, all of the reagents were used without additional purification or processing steps. Commercial ruthenium(IV) oxide (Alfa Aesar, anhydrous, 99.9%) was utilized as a standard reference for OER measurements. Details for the parallel synthesis of LaMnO3 nanorods are provided in the accompanying Supporting Information. We focus on LaNiO3 herein. Stage 1: Formation of La(OH)3−Ni(OH)2 Composite Nanorods. La(OH)3 and Ni(OH)2 were synthesized as follows. Commercial hydrated LaCl3 and NiCl2 were oven-dried at 130 °C for no less than 24 h to remove moisture for the purpose of achieving careful stoichiometric control. NiCl2 possesses a bright orange hue, and hence, its chemical transformation is normally registered as a perceptible color change. To 2 mL of a 0.9 M aqueous solution composed of dispersed LaCl3 and NiCl2 powders was added a single drop of concentrated HCl to prevent the formation of any insoluble lanthanum species, which would have increased turbidity within the clear green solution. A track-etched 200 nm pore-diameter polycarbonate template membrane was then immersed in the acidified solution and allowed to soak in solution to facilitate complete pore immersion and saturation with various metal ions. After the excess solution was drained, a 2.5 M NaOH solution was poured into the container to fully saturate and wet the inner pores of the polycarbonate template membrane. The subsequent precipitation of La(OH)3 and Ni(OH)2, as highlighted by eq 2 in Table 1, manifested itself as the formation of a white-to-pale green precipitate on the template surface immediately upon contact C


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Figure 1. (A) Powder XRD pattern of submicrometer intermediate nanorods formed in stage 1 along with reference patterns for Ni(OH)2 (no. 742075) and La(OH)3 (no. 83-2034). (B) TEM micrographs of typical nanorods generated from La(OH)3 and Ni(OH)2. The average diameter measured is 195.9 ± 20.3 nm. a JEOL 3000F microscope, equipped with a field emission gun operating at an accelerating voltage of 300 kV. 2.2.2. Electrochemical Characterization. Electrochemical characterization of the metal oxide catalyst was performed with the catalysts supported onto a glassy carbon electrode (GCE; 5 mm, Pine Instruments). After polishing the electrode with an aluminum oxide powder slurry (containing individual particle grains measuring 0.050 μm in diameter), 40 μL of an ethanolic dispersion of LaNiO3 (2 mg mL−1) was loaded onto the modified GCE. After the catalyst-loaded GCE was allowed to air-dry, one 5 μL drop of an ethanolic 0.025% Nafion solution was added to “seal” in the catalyst to the GCE surface. The GCE was subsequently immersed into DI water to remove either loose or soluble impurities. Electrochemical measurements were obtained in 0.1 M potassium hydroxide (Fisher Scientific, optima grade) solutions, created using high-purity water possessing a resistivity value of 18.2 MΩ cm. Pt foil and an Hg/HgO combination served as the counter and reference electrodes, respectively. Because absolute electrochemical values may vary depending upon a host of conditions including instrument configuration as well as on sample preparation techniques, we have also included a parallel set of results taken on commercially available RuO2 to serve as an experimental internal standard and control so as to ensure a meaningful relative comparison with our as-prepared ternary oxide samples. As such, the RuO2-decorated electrode has been prepared in a manner similar to that described for the analogous LaNiO3 samples. All potentials have been reported with respect to the reversible hydrogen electrode (RHE). The corresponding electrochemical properties of the catalysts were examined by linear sweep voltammetry (LSV) as well as using chronoamperometry (CA). Oxygen evolution reaction (OER) LSVs were collected at 1600 rpm in an oxygensaturated 0.1 M KOH electrolyte, using a scan rate of 10 mV s−1 within the potential range of 0.7−2.0 V versus RHE. The electrochemical results have been standardized to the geometric surface area of the electrode. Chronoamperometry measurements were acquired at 1.8 V vs RHE in a 0.1 M KOH electrolyte, after a duration of 180 min. All experiments have been repeated multiple times to ensure the reproducibility of the measured data.

formation of a white-pale green precipitate, which was subsequently allowed to “age” for 2 h at room temperature. A solid precipitate was collected upon centrifugation at 9000 rpm for 5 min to which a freshly prepared 2.5 M NaOH/1.25% NaClO solution was then slowly added. The use of sonication initiated powder dispersion. Within 2 min, the white-pale green powder had turned black. Sonication continued for an additional 10 min, and then the dispersion was allowed to “age” in the NaClO solution for a further 80 min without sonication, thereby giving rise to a total reaction time of 1.5 h. After these processing steps, the resulting black powder collected after centrifugation (1800 rpm for 10 min) was washed five separate times with deionized water. Upon drying at 80 °C for 1 h, the powder was then annealed in air inside a preheated tube furnace at 900 °C for 10 min after which the material was quenched to ambient conditions and then washed once more with DI H2O. The as-obtained, “shimmering black” LaNiO3 powder was mechanically compressed into a pellet within a 1 cm diameter die cast mold using a Carver laboratory press, operated at an isobaric pressure of 737 MPa at room temperature for 10 min. The resulting LaNiO3 pellet was not additionally mechanically processed. Rather, it was then annealed at 900 °C for another 10 min to promote crystallite sintering and growth, so as to ensure uniform chemical consistency within its bulk form. As such, “bulk” LaNiO3 (LNO-B) powder was obtained after the pellet had been air cooled to room temperature and then ground up, using a pestle and mortar. 2.2. Characterization. Morphological visualization of the assynthesized structures was accomplished via transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), energy-dispersive spectroscopy (EDS), and scanning electron microscopy (SEM) data. Surface area information was quantitatively achieved using a Quantachrome Nova 2200e surface area and pore size analyzer. Insights into their structural identity and chemical composition were provided by complementary powder X-ray diffraction, infrared (IR) spectroscopy, as well as X-ray photoelectron spectroscopy (XPS) data. Additional details concerning the standard experimental protocols used to collect the XRD, IR, surface area, XPS, and thermogravimetric analysis (TGA) data in particular are described in the Supporting Information. 2.2.1. Electron Microscopy. The various as-synthesized rods were analyzed using a field emission scanning electron microscope (FESEM Leo 1550) operating at 2.5 kV. To prepare these samples for structural characterization, fixed amounts were dispersed in ethanol and sonicated for ∼1 min, prior to their deposition onto a silicon (Si) wafer. Low magnification transmission electron microscopy (TEM) images were collected at an accelerating voltage of 120 kV with the JEOL JEM-1400 instrument, equipped not only with a 2048 × 2048 Gatan CCD digital camera but also with an Apollo XLTSUTW detector, which enables EDS mapping. Similar sample preparation protocols for electron microscopy analysis were initiated using an ethanolic dispersion coupled with sonication. Subsequently, the solution was deposited dropwise onto a 300 mesh Cu grid. Highresolution transmission electron microscopy (HRTEM) data and selected area electron diffraction (SAED) patterns were acquired with

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of La(OH)3− Ni(OH)2 Nanorods (Stage 1). The synthetic protocol utilized to generate LaNiO3 nanorods consists of three stages. To summarize, the reaction scheme incorporates steps including (1) precipitation, (2) chemical oxidation, and (3) annealing, respectively. Specifically, the template serves as the structural framework by either enabling the direct mixing of precursors within a spatially confined space or allowing for the localization of crystal growth, such that as-prepared nanostructures inevitably assume the shapes and size dimensions of the “originating” template itself. In other words, by eliminating the need for either superfluous surfactants or chemical growthD


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Figure 2. (A) Bright field image captured in STEM mode of intermediate La(OH)3-Ni(OH)2 rods. Elemental signals ascribed to O, La, and Ni are respectively highlighted in (B), (C), and (D) for this sample region. The individual isolated elemental mapping data are overlaid in (E). The affixed scale bar measures 1 μm in each of these images. (F) STEM image for an area designated for associated line scan elemental analyses, shown in (G).

directing agents, the use of a polycarbonate (PC) template as a geometric scaffold allows for a simple, straightforward, robust, and reproducible synthetic procedure. For the generation of nanowires, commercially available PC membranes, in particular, feature a reasonably high density of parallel, straight, nanosized cylindrical pores characterized by a uniform diameter. Moreover, as-prepared nanowires can be easily isolated either (i) by selectively dissolving the PC template with the use of an organic solvent such as dichloromethane or (ii) by polymer template combustion during a postprocessing annealing step, for instance. Mid-IR spectroscopy (Figure S1) reveals that most of the carbonaceous template had actually been removed after washing with DCM. As per eq 2 in Table 1, uniform hydroxide-based precursor 1D rods were synthesized in the initial stages of the reaction in which the solvated metal ions precipitate as Ni(OH)2 and La(OH)3, because both nickel and lanthanum hydroxide possess negligible solubility in water. In effect, Figure 1A illustrates the powder XRD profile, confirming the production of both Ni(OH)2 and La(OH)3. We deliberately chose 2.5 M NaOH as an optimal concentration to initiate chloride conversion and metal hydroxide precipitation under roomtemperature conditions, because the use of a higher NaOH concentration might have resulted in possible structural damage to the template pores. Figure 1B shows a typical TEM image of representative rod-like structures of our samples. Figure S2A highlights the apparent morphological state of the metal hydroxide-filled template after base treatment. Despite sonication efforts to remove surface contamination, unwanted irregularly shaped deposits remain, and may require a more direct method of physical removal, such as “grit” polishing. We found that “2000 grit” sandpaper possesses a fine enough grain size distribution that allowed for us to adequately abrade away exterior surface deposits, ranging in size from micrometers to a few hundred nanometers, without physically tearing apart the template itself. Figure S2B provides evidence for the relative smoothness of the surfaces of the

template, after physical abrasion with the sandpaper, thereby confirming the efficacy of our protocol. The entrapped metal hydroxide rods were subsequently freed and isolated after removal of the 200 nm pore-size templates themselves via dichloromethane solvation followed by washing. Figures 1B and 2A−F illustrate the morphology of these structures, possessing average diameters of 195.9 ± 20.3 nm, consistent with the 200 nm pore diameter template used. The nanorods imaged under TEM appeared to give rise to the same degree of contrast, thereby suggesting the likelihood of longrange compositional homogeneity and order. Furthermore, EDS mapping, as specifically depicted in Figure 2A−E, confirms that all of the expected constituent, component elements (i.e., La, Ni, and O) are homogeneously and evenly distributed throughout the as-formed nanorods themselves. The line scan profile (Figure 2F and G), with data taken at 5 nm intervals along a given 580 nm distance, designates similar and consistent intensities for all three elements spatially dispersed along a representative length of a single individual nanorod. We assert that it is unlikely that a solid solution has formed, due to a number of factors including but not limited to (a) distinctive variations in the space group, because La(OH)3 possesses a space group comprised of hexagonal P63/m (JCPDS no. 83-2034), whereas Ni(OH)2 maintains P3̅m1 (JCPDS no. 74-2075) symmetry elements; (b) incompatible differences in atomic radii, because La3+ is characterized by a radius of 103.2 pm, whereas Ni2+ is much smaller with a radius of 69 pm; and finally, (c) valence charge dissimilarities associated with the juxtaposition of La (+3) versus Ni (+2) species. This hypothesis is substantiated and confirmed, when comparing the XRD patterns of our as-synthesized metal hydroxide nanorods with the corresponding reference patterns for α-Ni(OH)2 and La(OH)3, respectively (Figure 1A). Moreover, the combination of TEM data backed up with EDS and XRD analyses suggests that the rods are indeed composed of small constituent, phase-segregated Ni(OH)2 and La(OH)3 domains. E


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Figure 3. (A) High- and (B) low-magnification SEM images of a typical 1D La(OH)3−NiOOH intermediate (LNO-Intermediate) structure, after chemical oxidation.

Table 2. Quantitative Diameter and Accompanying Chemical Composition Data Provided for a Series of As-Synthesized LaNiO3 Nanorods (i.e., LNO-600, LNO-700, LNO-800, and LNO-900, Prepared under Annealing Temperatures of 600, 700, 800, and 900 °C, Respectively), in Addition to Bulk (LNO-B) sample of as-prepared nanorods and of bulk

average diameters measured with standard deviations (in nm)

experimental elemental ratio of Ni3+:Ni2+ (XPS data)

La(OH)3-Ni(OH)2 La(OH)3-NiOOH LNO-600 LNO-700 LNO-800 LNO-900 LNO-B

195.9 ± 20.3 185.2 ± 16.1 141.5 ± 9.5 125.1 ± 10.5 92.4 ± 12.4 99.1 ± 10.2 2−13 μm (aggregate dimensions), 194.3 ± 42.3 (dimensions of constituent particles)

65.2:34.8 81.6:18.4 84:16 89.3:10.7 90.6:9.4 95.1:4.9

3.2. Synthesis and Characterization of La(OH)3− NiOOH Composite Nanorods (Stage 2). To ensure the complete oxidation of Ni2+ to Ni3+, a separate reaction step was introduced, wherein the Ni(OH)2 component fabricated in stage 1 was chemically oxidized to NiOOH using NaClO,44 as described by eq 3 (Table 1). That is, the as-prepared metal hydroxide composite (rod) powder was dispersed in a solution consisting of 1.2% NaClO and 2.5 M NaOH. NaOH was used to stabilize NaClO(aq) in solution, because NaClO(aq) is known to decompose over time.45 On the basis of our unpublished studies, we have found that chemical oxidation of Ni2+ at room temperature represents an optimal means for enabling a facile annealing procedure that was least likely to cause sintering of the precursor rods. In fact, we assert that evidence for successful Ni2+ oxidation was provided by a substantial darkening in the visual hue of the dispersion, coupled with the disappearance of the Ni(OH)2 peaks in the powder XRD profile. These observations are consistent with the formation of a La(OH)3−NiOOH composite nanorod intermediate (Figure S3). Figure 3A and B highlights our as-synthesized “LNOIntermediate” species, which had been created from the chemical oxidation of precursor metal hydroxide rods and which possess an average diameter of 185.2 ± 16.1 nm. Although a slight change in the diameter of the resulting expected rods was observed, relative to that of the precursor metal hydroxide rods, the morphology of the “final” rods does not appear to have discernibly changed. Furthermore, the rods themselves are reasonably homogeneous and uniform in size. 3.3. Synthesis and Characterization of LaNiO3 Nanorods (Stage 3). The final stage of the synthesis entails annealing the “LNO-Intermediate” at either 600, 700, 800, or 900 °C, respectively, for 10 min in air in a preheated tube furnace, subsequently followed by quenching in air. As quantitative confirmation for the validity of our chosen annealing temperature, Figure S4A and the accompanying discussion collectively illustrate and describe a typical TGA weight-loss profile associated with the precursor “LNO-

Intermediate” composite material. Moreover, to determine the effect that varying the annealing temperature has upon the formation of oxygen vacancies within our samples, the extent of which was quantitatively estimated using a previously reported protocol,46−48 we performed TGA analysis on all of our assynthesized products, comprising samples of LNO-600, LNO700, LNO-800, LNO-900, and LNO-B (Figure S4B−F). Specifically, we completely oxidized a few milligrams of each product in air at 900 °C with the subsequent observed increase in mass, likely ascribable to an excess of oxygen content being incorporated within the metal oxide lattice, which would have been responsible for oxidizing Ni2+ to Ni3+. This measurable mass increase could therefore be used to indirectly estimate the level of oxygen vacancies, actually present within our individual samples. As such, our approximate calculations on the likely stoichiometries of our as-prepared LNO-600, LNO-700, LNO800, LNO-900, and LNO-B samples yielded LaNiO2.95, LaNiO2.96, LaNiO2.98, LaNiO3, and LaNiO3, respectively, thereby confirming that more oxygen vacancy defect sites are expected to have been generated at lower annealing temperatures. This is certainly a reasonable assertion, because higher annealing temperatures are presumably more conducive to eliminating defects. Table S1 summarizes these findings. A similar method of vacancy analysis has been used with respect to the study of other comparable metal oxide systems, such as SrTiO3,47 Sr0.95Ti0.9Nb0.1O2.90,49 Mn-doped SrTiO3,50 and (La,Sr)CoO3.51 Figure S5A−D represents typical low-magnification TEM micrographs, illustrating how morphology alters as a function of increasing annealing temperatures. Specifically, as the annealing temperature is increased, the rods appear to become less rigid, tend to aggregate, and seem to contract in size to some extent, as is evident from the narrower diameters measured in Table 2. In terms of why the diameters of the nanorods may have reduced after annealing, this observation is anticipated, because the intermediate hydroxide species from stages 1 and 2 are associated with an entirely different compositional species as F


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ACS Applied Materials & Interfaces compared with the final isolated LaNiO3 perovskite product. Specifically, the LNO-Intermediate sample of stage 2 would be expected to have dehydrated and to have lost mass in the form of H2O, upon conversion to LaNiO3. Moreover, higher magnification TEM micrographs (Figure 4) highlight the

Figure 5. Powder XRD pattern for the as-prepared LaNiO3 samples, annealed at 600, 700, 800, and 900 °C, respectively, as well as of the synthesized bulk. The acquired profiles match well with that of the LaNiO3 database standard (JCPDS no. 34-1028).

Figure 4. Higher magnification TEM micrographs of as-synthesized LaNiO3 nanorods annealed at (A) 600 °C, (B) 700 °C, (C) 800 °C, and (D) 900 °C, respectively, for 10 min.

accompanying change in the surface texture of the nanorods, as a function of annealing temperature. For instance, the rods appear to distort and deform with increased annealing and sintering from 600 to 900 °C. Hence, we chose the “optimized” annealing temperature of 600 °C to ensure not only preservation of a discrete morphology but also complete chemical conversion to crystalline LaNiO3, a process depicted in eqs 4−6 (Table 1). In doing so, we were able to minimize sintering and the associated loss of active catalytic surface area. We emphasize that, although the annealing process itself is brief (i.e., 10 min), it remains sufficient to generate rhombohedrally centered hexagonal R3̅c (JCPDS no. 34-1028) LaNiO3, as per the reference patterns highlighted in Figure 5. This finding was independent of annealing temperature considerations, because even at 600 °C, the lowest annealing temperature used in our studies, we were in fact able to isolate crystalline and pure product. Also worth noting is that the degree of crystallinity of the rods appears to increase as a function of correspondingly rising annealing temperatures. Indeed, the Debye−Scherrer estimates for crystallite sizes for LNO-600, 700, 800, and 900 are 8.6, 9.6, 13.3, and 17.3 nm, respectively. Figure 6A−D represents typical SEM micrographs of all of the as-generated nanorods. LNO600 possesses the largest observed diameter (i.e., 141.5 ± 9.5 nm) measured of the samples analyzed, and this sample is expected to have only minimally sintered, because its annealing temperature is the lowest. Not surprisingly, LNO-600 is also characterized by a relatively roughened profile. Figure S6 offers additional insight into the overall diameter reductions associated with increasing annealing temperatures, which are quantitatively summarized in Table 2. The XRD

Figure 6. SEM micrographs of typical LaNiO3 nanorods, annealed at (A) 600 °C, (B) 700 °C, (C) 800 °C, and (D) 900 °C, respectively. The overall 1D shape of the rods is maintained after annealing. (E) EDS spectra provided for LaNiO3 nanorods annealed at 600, 700, 800, and 900 °C (samples LNO-600, LNO-700, LNO-800, and LNO-900, respectively), as well as of as-synthesized LaNiO3 bulk (sample LNOB).

patterns in Figure S6 suggest that, relative to samples prepared at lower annealing temperatures, the use of higher annealing temperatures brings about a shift of the (110) peaks toward a larger 2θ, which is indicative of a lattice contraction. One hypothesis for this observation in agreement with our XPS data is that increased annealing may induce the more complete conversion of Ni2+ point defects to smaller Ni3+ species. The implication is that our perovskites may incorporate greater quantities of oxygen vacancies at correspondingly lower annealing temperatures. Figure 7A features a high-resolution TEM micrograph image of a typical nanorod. Figure 7B highlights the ordered arrangement of the constituent crystallites within the LaNiO3 G


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Figure 7. (A) Lower and (B) higher magnification HRTEM micrographs of individual, as-prepared LaNiO3 nanorods, along with the corresponding (C) SAED pattern. The d-spacing measured in the high-magnification image can be ascribed to LaNiO3 (110).

Figure 8. (A) Low-magnification SEM and (B) corresponding high-magnification SEM images of as-synthesized bulk LaNiO3.

rates the presence of the above-reported elements, along with atmospheric carbon contamination (Figure S8). Using a similar synthetic procedure, analogous LaMnO3 nanorods have been synthesized. Panels within Figure S9 illustrate (A) “high aspect ratio” intermediate La(OH)3 and Mn(OH)2 composite rods, (B) LaMnO3 rods synthesized at 600 °C for 10 min, and (C) LaMnO3 rods fabricated at 800 °C for 10 min. The corresponding XRD profiles for LMO-600 and LMO-800 are shown in panel (D) with LMO-600 and LMO800, denoting LaMnO3 samples generated at 600 and 800 °C, respectively. We note that the XRD profiles are in generally good agreement with the reference pattern, JCPDS no. 852219, for rhombohedrally centered hexagonal LaMnO3, characterized by a space group of R3c. Table S2 summarizes the observed diameters of all of these LaMnO3 nanorod-like structures. 3.4. Synthesis and Characterization for Bulk LaNiO3. To reinforce the advantages of the nanoscale 1D motif of LaNiO3, we synthesized the corresponding LaNiO3 bulk using a protocol similar to that utilized for producing the nanorods themselves. The procedure differs somewhat from the actual nanorod synthesis in that (i) the particle size is not dictated by the spatial confines of any template pore channels and (ii) crystal domain growth is allowed to proceed without greatly increasing the annealing duration. This objective was achieved by subjecting the sample to a relatively high applied isobaric pressure of 737 MPa under ambient temperature for 10 min to promote tight crystallite packing. This was followed by subsequent annealing to sinter and weld the individual crystallites together so as to increase the average observed crystal size with minimal annealing. Figure 8A and B presents SEM images of the bulk sample and illustrates the presence of larger, sintered aggregates measuring 2−13 μm in diameter, which are composed of smaller individual constituent units averaging 194 ± 42 nm in diameter, coalesced together. The corresponding XRD pattern

nanorods themselves. These unit crystallites consist of welldefined planes, which correlate with the expected LaNiO3 (110), characterized by a measured d-spacing of 2.67 Å. This value is within experimental error of the theoretical d-spacing for the (110) plane, which is 2.72 Å (JCPDS 34-1028). Whereas these planes are pronounced, the realization of overall monocrystalline continuity is thwarted by the grain growth associated with neighboring crystallites, an observation that is consistent with polycrystallinity. A SAED pattern can be found in Figure 7C, with the diffraction rings indexed. This SAED pattern corroborates the idea of polycrystallinity, as the observed rings highlight a number of planes possessing lower Miller indices, such as the (110) plane. Figure S7 shows the accompanying lower magnification HRTEM image of this particular nanorod that we had specifically analyzed. Qualitative EDS data provided in Figure 6E suggest that the only elements present in appreciable quantities in the sample are La, Ni, and O with the Si signal emanating from the underlying wafer. Significantly, there is no observable C signal. These elemental analysis results corroborate the IR data (Figure S1), which collectively suggest that after annealing at 600 °C, the majority of the carbon has been removed and is no longer present on the nanorod surface. That is, we do not see any perceptible traces of either carbon or carbon-containing functional groups, which would have been indicative of the lingering presence of a template. Moreover, we do not observe any detectable Fe signal either. This is also an important finding, because Fe impurities have been previously shown to enhance Ni-driven OER processes. 52 A more careful quantitative analysis53 of the EDS data implies that the Fe quantity is no more than ∼1%, and as such, if present at all, would only be found in trace amounts. In addition, we did not discern any appreciable quantities of Cl− ions either, implying that the washing steps both during and after the synthesis process appear to be capable of removing any residual precursor constituents. A survey XPS scan similarly corroboH


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Figure 9. (A) Oxygen evolution activity of our as-synthesized LaNiO3 nanorods with a 40 μL catalyst loading, as measured by the rotating disk electrode run at 1600 rpm in an O2-saturated 0.1 M KOH solution, operating at 10 mV s−1. Commercial RuO2 NPs were also tested as a comparative standard with the identical 40 μL loading. (B) Chronoamperometry measurements of our as-prepared LaNiO3 nanorods, bulk LaNiO3, as well as commercial RuO2 (i.e., same 40 μL loading) were taken at 1.8 V vs RHE in 0.1 M KOH over a testing duration period of 180 min.

To highlight the overall importance of the 1D anisotropic motif, we compared the performance of our LaNiO3 nanorods with the corresponding bulk sample under identical experimental electrochemical conditions. To summarize, all of our nanorods definitively outperformed the corresponding, assynthesized bulk control sample at all potentials, in the range of 0.7−2.0 V, as summarized in Table 3, in terms of the current density. At the same time, most of our nanorod samples yielded a lower onset potential as compared with the bulk control sample, indicative of better reaction kinetics. Interestingly, the

in Figure 5 evinces very good crystallinity, whereas the Brunauer−Emmett−Teller (BET) surface area analysis (Figure S10A and B) reveals a surface area of 4 m2 g−1, which is lower than the corresponding value for LNO-600 (i.e., 25 m2 g−1). Table S3 provides for various reported values of surface area measurements of as-prepared LaNiO3. The results obtained from SEM, XRD, and BET collectively suggest that the bulk maintains the expected LaNiO3 chemical composition, albeit with a different and distinctive morphology, thereby denoting the perfect comparative sample with analogous LaNiO3 nanorods for OER analysis. 3.5. Observation of and Mechanistic Insights into the Oxygen Evolution Reaction Activity Results of Our LaNiO3 Nanorods. To test the viability of employing our as-synthesized LaNiO3 nanorods as OER catalysts, these nanomaterials were rotated at 1600 rpm in an O2-saturated 0.1 M KOH electrolyte at 10 mV/s between 0.7 and 2.0 V versus RHE, as can be seen in the linear sweep voltammetry curve shown in Figure 9A. For our experiments, we used an alkaline 0.1 M KOH solution, in part because basic conditions have been shown to yield increased OER performance for various metal oxide catalysts.54−56 We noted that the asprepared LaNiO3 gave rise to an OER onset potential of ∼1.6 V vs RHE. The sharp increase in observed electrochemical activity after 1.6 V is likely due to the evolution of oxygen and is similar to that observed with other studies of LaNiO3.34,57 In addition, with commercial RuO2 nanoparticles, we recorded an analogous OER onset potential of 1.48 V vs RHE.

Table 3. OER Performance Summary for All of Our AsPrepared LaNiO3 Nanorods (Generated at Annealing Temperatures of 600, 700, 800, and 900°C, Respectively), Bulk LaNiO3, and Commercial RuO2 sample material RuO2 LNO600 LNO700 LNO800 LNO900 LNO-B I

onset voltage (vs RHE)

overpotential measured at 10 mA cm−2 (in V)

current density (in mA cm−2) obtained at 1.8 V (vs RHE)

1.46 1.56

0.456 0.572

17.5 10.0

1.57

0.583

9.4

1.61

0.630

7.1

1.61

0.633

6.7

1.61

0.676

6.1


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Figure 10. XPS spectra of the Ni 3p peak associated with (A) the intermediate sample, “LNO-intermediate”, in addition to a series of as-synthesized LaNiO3 samples, specifically, (B) LNO-600, (C) LNO-700, (D) LNO-800, (E) LNO-900, and (F) bulk LNO-B, generated using various annealing temperatures.

versus 700 °C, specifically, LNO-600 versus LNO-700; (B) LNO-900 versus LNO-B; and (C) LNO-600 versus LNO-B. To provide a relevant context for our discussion, we note the following key points. The formation of oxygen vacancies appears to be more prevalent at lower temperatures. We gain this understanding, based upon a number of careful observations. Specifically, (i) we obtained TGA data (Figure S4) on our key intermediate product, namely, La(OH)3− NiOOH, which had been systematically heated and annealed in air. An evident increase in mass was observed until ∼900 °C, wherein the rate of mass change became negligible. This last observation in the TGA profile can be interpreted as the oxidation of the remaining Ni2+ to Ni3+, after the majority species of LaNiO3 had already formed. This finding is corroborated by and consistent with our quantitative TGA results described earlier, in which we found that the sample annealed at the lowest temperature of 600 °C (i.e., LNO-600) gave rise to the most oxygen vacancies (i.e., a calculated composition of LaNiO2.95), whereas the samples annealed at the highest temperature of 900 °C (i.e., LNO-900 and LNO-B) yielded the correspondingly least number of vacancies (i.e., a computed composition of LaNiO3). We also detected (ii) a clear XRD shift in the (110) peak of the LNO-700 sample, relative to that of LNO-600, toward higher 2θ, as illustrated in Figure S6. It is worth noting that each sample was prepared from an ethanolic slurry, which was drop-cast onto the Si-based zero background holder and later dried until a reproducibly thin immobilized film formed. Additionally, the holder was mechanically immobilized within the instrument, thereby further minimizing the chance that the observed peak shift was due to a sample processing artifact. As

highest performing nanorods we tested were those synthesized with an annealing temperature of 600 °C, followed by the analogous samples created at 700, 800, and 900 °C, respectively. That is, activity-wise, LNO-600 > LNO-700 > LNO-800 > LNO-900, in that specific order. Our results evidently show that our nanorods exhibit higher activity as a function of a general decrease in the corresponding annealing temperature. This observation would appear to contradict the trend observed by Zhou et al.,34 who found that an increase in the annealing temperature of LaNiO3 clearly correlated with a corresponding rise in OER activity. We reasonably hypothesize that this seeming contradiction may primarily stem from the use of two very different synthetic protocols, consistent with our emphasis herein on the ability of chemistry to dictate electrochemical behavior. In effect, our synthesis protocol relies on a three-step process, incorporating coprecipitation, chemical oxidation, and thermal annealing steps, whereas Zhou et al. relied on a potentially simpler sol− gel complexation method.58 Hence, a direct comparison of the products of such disparate treatments is more complex and nuanced. Therefore, to gain thoughtful and meaningful insights into our own OER performance results, we would like to discuss and rationalize our data, based upon considerations of morphology-dependent surface area (SA) and the oxidation state of surface Ni as key determining parameters. The individual importance and interconnectivity of these various factors becomes apparent, when explicitly comparing the following “case studies” of samples for internal consistency: (A) a comparison between the nanorods, prepared at 600 J


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occupancy in the eg orbital increases, as Ni2+ is generated within the crystal lattice. Therefore, the observed enhancement in OER activity in our series of as-prepared “high aspect ratio” samples (especially with the highly oxygen-deficient LNO-600) as compared with the analogous bulk is likely a result of a greater proportion of Ni2+ species (or, conversely, an increased oxygen vacancy) within our nanorods with decreasing annealing temperatures. This scenario leads to a slight increase in the eg orbital occupancy, because the lower oxidation state of Ni2+ possesses more d electrons than Ni3+. Such a trend is consistent with the established notion of an ideal eg occupancy filling, which lies just above 1. A comparison between the LNO-900 and the bulk (LNO-B) samples, namely, case B, is significant, because the annealing temperatures between the two are identical and hence “normalized”. Indeed, the main difference between these two samples is with respect to morphology. Specifically, at 1.9 V (vs RHE), our nanorods exhibit a current density of 12.0 mA cm−2, whereas the bulk sample maintains an analogous current density of 9.5 mA cm−2. Interestingly, bulk-sensitive TGA analysis determined that the overall Ni2+ content is quite similar in these samples with measurements of 0.2% and 0.1% Ni2+ associated with the LNO-900 and LNO-B samples, respectively. Conversely, Ni 3d fittings from our surface-sensitive XPS analysis (Figure 10) showed a much larger (i.e., almost double) surface compositional difference of 9.4% versus 4.9% Ni2+ content for the LNO-900 and LNO-B samples, respectively. Hence, it is likely that the morphology-induced, higher exposed surface area associated with the LNO-900 nanorods allows for a much greater surface reactivity as compared with the LNO-B (bulk) sample. In effect, with the LNO-900 sample, the greater quantities of surface Ni2+ species likely account for the improved OER performance with respect to bulk. As a final point of relevant comparison, with case C, we find that the LNO-600 sample clearly outperforms its LNO-B analogue. Indeed, at 1.9 V, wherein LNO-600 delivers a current density of 15.8 mA cm−2, LNO-B yields a corresponding current density value of 9.4 mA cm−2. We note that the LNO600 sample possesses not only significantly more surface Ni2+ species but also a higher surface area, as compared with LNOB. Specifically, both the XPS and the TGA results are collectively consistent with the idea of a greater surface Ni2+ content within LNO-600 as compared with LNO-B. Hence, LNO-B should maintain fewer oxygen vacancies as compared with the LNO-600 sample. Moreover, from BET measurements, LNO-600 is characterized by a surface area of ∼25 m2 g−1, whereas that of LNO-B measures ∼4 m2 g−1, which is a factor of 6 smaller. Nevertheless, the observed electrochemical activity difference, entailing a factor of 1.7×, is comparatively smaller than expected with respect to their corresponding surface area variance. Hence, the important and intriguing issue is to explain the unexpectedly high performance of the bulk sample. One plausible idea is associated with the greater metallic character of an “oxygen-rich” LaNiO3 lattice. Studies have suggested that oxygen vacancies in LaNiO3 do not necessarily create mobile carriers but rather provide a source of electrons to localized states, thereby generating Ni2+ cations,64−66 which can reduce the overall electronic conductivity of the material. This is a significant point, because LaNiO3 is understood to be metallic, wherein its metallic component lies within its O−Ni−O framework.67,68

such, the gradual shift observed for the (110) peak position associated with each sample can most likely be ascribed to an intrinsic property of the sample itself. Perovskites in general are known to facilitate reductive nonstoichiometry, wherein a substantial amount of oxygen vacancies can be created due to the presence of “redox-sensitive” and potentially “reducible” metal ions within the lattice. This scenario has been elaborated upon in the case of LaMnO3, wherein lattice expansion is observed with the concomitant formation of oxygen vacancies.59 Last and most important, a higher degree of oxygen vacancy is suggested by (iii) quantitative XPS data collected on all of the LaNiO3 samples. All samples show a general position of the Ni 3p peak within the range of 67.2−67.6 eV, which is in good agreement with the reported literature values of 67.6 and 68.29 eV60,61 nominally associated with Ni3+ 3p binding energies. Table 2 lists the experimental XPS-derived ratios of Ni3+:Ni2+ species for each of our as-prepared samples as well as for the “LNO-Intermediate”. Figure 10 highlights the Ni 3p fitting spectra, from which these ratios were obtained. With increasing annealing temperature, it is apparent that the proportion of Ni3+ species correspondingly and steadily increases from 81.6%, 84%, 89.3%, and finally to 90.6% for LNO-600, LNO-700, LNO-800, and LNO-900, respectively. Moreover, the LNO-B contains the most amount of Ni3+ at 95.1%. Interestingly, TGA data exhibit a similar albeit more pronounced trend, in which the proportion of Ni3+ similarly rises with ever higher annealing temperatures. For example, the percentage of Ni ions in the 3+ oxidation state for LNO-600 is calculated to be 81.6% by XPS but as high as 90.9% from the TGA analysis. A likely explanation for this discrepancy is that the XPS analysis is particularly sensitive to the external surface composition, whereas TGA analysis (Table S1) is more attuned to the bulk of the material. Altogether, these complementary techniques suggest that there is indeed a finite Ni 2+ concentration throughout the crystal with a decidedly higher localization at the surface of the crystal, relative to the sample interior. For case A, the nanorods (LNO-600 vs LNO-700) possess the same morphology but are dissimilar in terms of the precise amount of Ni2+ content within their respective crystallographic lattices. LNO-600 outperforms LNO-700. Therefore, to compare their OER performance is to highlight the significance of Ni2+ defect formation. To emphasize this argument, we analyzed and compared their activities, acquired at an applied voltage of 1.9 V (vs RHE). In effect, the higher activity of LNO600 (measured at ∼15.8 mA cm−2) as compared with that of LNO-700 (observed at ∼14.4 mA cm−2) and even LNO-900 (found to be ∼12.0 mA cm−2), for example, can best be explained in terms of differences in Ni valency. Specifically, in the context of OER, the presence of oxygen vacancies in LaNiO3 is significant, because vacancies are normally associated with the concomitant generation of Ni2+ species, so as to ensure overall electroneutrality. Suntivic et al. reported on the activity of various perovskite oxides, noting a volcano-type correlation between the eg orbital occupancy and the OER catalytic performance,62 wherein peak catalytic performance was anticipated when the eg orbital filling was slightly greater than 1. It is known that the Ni3+ in LaNiO3 typically features a “low spin” electron configuration having 6 electrons within the t2g orbital, while only 1 electron occupies the eg orbital at room temperature.63 This level of electron K


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resulting in a loss of catalyst mass from the electrode. Binninger et al. have reported that the essential driving force for metal oxide corrosion is rooted in the inherent instability of the oxygen anion within the oxide lattice.72 Interestingly, it has been posited that removal of gas bubbles from the surfaces of the RuO2 during the OER process can ultimately lead to improved stability by precluding the generation of these more oxygen-rich (and more soluble) Ru species.73 By contrast, the performance loss with respect to all of our LaNiO3 samples tested may likely be ascribed to a decrease in the number of favorable electrochemically active sites, as shown in Figure S11. To account for this possibility, surface reconstruction, morphological degradation, aggregation, and agglomeration all may have occurred to some extent, thereby resulting in a loss of exposed active surface area. Tafel slopes for the best-performing LNO-600 sample and the commercial RuO2 have been computed from their respective linear sweep voltammetry data, as shown in Figure S12. The curves span the potential region wherein OER is known to be occurring. It has been previously asserted that the Tafel slopes for LaNiO3 are highly dependent upon the synthetic method employed.34,74,75 Our data suggest that, although the initial specific activity for commercial RuO2 is higher than that of our as-synthesized LaNiO3 nanorods, our as-prepared sample still shows a very pronounced increase in activity, and, moreover, our data are quantitatively within the range of prior reports on these systems.34,74,75 We emphasize that what is highly significant about our data is that with only a total perovskite catalyst loading of 0.08 mg, and without the addition of any carbon, which would have increased the overall conductivity of the catalyst, we were able to report on a measured current density of 10 mA cm−2 at 1.8 V versus RHE, which is a respectable result. By contrast, the use of RuO2 with an identical mass loading (i.e., 0.08 mg) yielded a current density of 17.4 mA cm−2. The main point is that, as compared with RuO2, LaNiO3 can achieve a very similar activity with only a slightly increased catalyst loading but at a substantial cost reduction. Specifically, on the basis of the current market prices for Ru (∼$1400 per kg),76 La ($7 per kg),77 and Ni (∼$9 per kg)77 metals, respectively, it is evident that our perovskite catalyst is significantly more cost-effective, while functionally capable of yielding almost an equally attractive set of performance metrics. Hence, the proven activity and noteworthy stability of our nanorod catalysts toward OER, coupled with the relatively abundant supply of La and Ni, render LaNiO3 as a potentially promising candidate for OER-driven devices, such as Li−air and Zn−air batteries. This finding is especially important, because the best performing catalysts to date are IrO2 and RuO2. These latter binary oxides contain expensive precious metals (Ir and Ru), thereby rendering them as particularly costineffective. Moreover, the activity data and accompanying stability of our perovskite nanomaterials herein demonstrate reasonably comparable OER performance, normalized by mass loading, with one of the highest performing metal oxides known to date, namely, commercial RuO2, in the absence of conductive additives such as carbon, the latter of which is specifically known to greatly enhance OER performance.

Specifically, the underpinning of the metallic nature of LaNiO3 is associated with the strong hybridization between the Ni3+ 3d and the O 2p orbitals.69 As compared with the 3d orbital of Ni3+, the 3d orbital of Ni2+ yields less of an overlap with the O 2p orbital, because it is known that electronegativity scales with both the ionization energy of a transition metal and the hybridization in perovskite oxides.63 That is, the lower oxidation state of Ni2+ maintains not only a lower electronegativity but also a decreased ionization energy as compared with Ni3+. This reduction in ionization energy associated with replacing Ni3+ with Ni2+ results in an increased metal d electron localization, because the Ni d-band would be expected to move away in energy from that of the O 2p orbital.63 Therefore, an overall reduction in metallic character would be predicted with oxygen-deficient, Ni2+-rich LaNiO3(−δ) (i.e., LNO-600) as compared with comparatively more oxygen-rich, more Ni2+deficient LaNiO3 (i.e., LNO-B). That is, we posit that LNO-B performs better than expected, because it is more electrically conductive than its more oxygen-deficient LNO-600 counterpart. In light of all of these data, it seems that to optimize LaNiO3’s OER performance, in the absence of conductive carbon support, requires a delicate balance between opposing trends: (i) favorable enhancements derived from having a “near ideal” eg orbital filling versus (ii) the inherently lower conductivity of a “Ni2+-deficient” LaNiO3 lattice. The latter point may be mitigated by using an underlying conductive carbon network, as is commonly done for metal oxide-based catalyst systems. Table 3 summarizes the OER performance of all as-synthesized materials as well as of RuO2. In addition, Table S3 highlights OER performance values for various LaNiO3 systems, reported in the prior literature. 3.6. Complementary Chronoamperometry Data. Moreover, we also compared the relative electrocatalytic stabilities of the LNO-600 sample, the LaNiO3 bulk, and commercial RuO2 in Figure 9B. The relevant insights into stability were provided using chronoamperometry, as is generally performed for comparable OER catalyst systems.54,56,70 Specifically, samples were run at a potential of 1.8 V over a 180 min duration. Initially, the order of decreasing current density followed the trend of commercial RuO2 > LNO-600 > bulk LaNiO3. Nevertheless, if one looks closely at Figure 9B, it is apparent that the current density of LNO-600 noticeably decreases (i.e., 11 mA cm−2 down to 1 mA cm−2 after 3 h). However, the comparative stability of commercial RuO2, used as a control and denoting the conventional “stateof-the-art” in the field, is much worse with a massive “drop off” in performance (i.e., 16 mA cm−2 down to ∼0 after 3 h), evident even after 40 min of testing. Hence, we demonstrated that our as-prepared samples were more durable than the corresponding commercial sample, because even after 30 min, the commercial RuO2 had degraded considerably more than our LNO-600 sample. Of significance, after a reasonably long duration of 180 min of testing, the resulting observed trend in current was characterized by LNO-600 > bulk LaNiO3 > RuO2. In general, in addition to the observed OER activity, metal oxide stability under typical OER conditions remains an equally significant concern. In the case of the degradation of commercial RuO2 catalysts, the Pourbaix diagram for Ru shows that under highly alkaline conditions, at high potentials, both anionic RuO4− and possibly HRuO5− ions can potentially replace RuO2 as the most stable species present.71 However, these species tend to dissolve within the solvent, thereby

4. CONCLUSIONS In this Article, we have primarily focused on a fundamental mechanistic understanding of the precise synthesis and preparation of novel as-prepared catalyst materials. Second, L



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ACKNOWLEDGMENTS Research for all authors was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. Experiments for this Article were performed in part at the Center for Functional Nanomaterials located at Brookhaven National Laboratory, which is supported by the U.S. Department of Energy under contract no. DE-SC00112704.

we have sought to correlate structure and formation conditions with the resulting OER properties observed. In effect, our objective has been to provide useful insights into the basic chemical underpinnings of how perovskites form and their subsequent role in electrocatalysis. Specifically, we describe the synthesis and characterization of lanthanum nickel oxide (LaNiO3) nanorods prepared using (a) a simple and novel template-assisted coprecipitation method of lanthanum hydroxide−nickel hydroxide precursors followed by (b) the mild annealing treatment of as-prepared lanthanum hydroxide−nickel oxyhydroxide composite nanorods. The generalizable protocol allows for significant synthetic flexibility in the sense that by simply using templates, possessing variously known pore diameters, as the spatially confining reaction environment, nanorods of a specific and reasonably monodisperse range of sizes can be correspondingly generated. Moreover, one need only vary the identity of chemically compatible metal precursors, diffusing within the template pore channels, to produce pure, crystalline perovskite nanorods of a desired and well-defined chemical composition. We have demonstrated the feasibility and practicality of this idea in our successful parallel synthesis of LaMnO3 nanorods. In the absence of an underlying conductive carbon support, our as-synthesized LaNiO3 rods have proven to be electrochemically active for the oxygen evolution reaction with an onset of 1.56 V vs RHE and an activity of 10.0 mA cm−2 measured at 1.8 V vs RHE with a 0.08 mg loading, denoting improved performance relative to bulk. In terms of mechanistically explaining the as-obtained results, it is likely that samples prepared at lower annealing temperatures were electrochemically superior to those created at higher temperatures, due to the presence of more oxygen vacancies and the concomitant formation of catalytically active Ni2+ species localized at the surfaces of our anisotropic LNO nanowires.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06855. Additional introduction, experimental procedures, detailed synthetic protocols associated with the formation of both LaNiO3 and LaMnO3, as well as physicochemical characterization of both LaNiO3 and LaMnO3 intermediates and products, including TGA curves, IR and XPS spectra, surface area analysis, representative SEM and TEM images, relevant electrochemical data for all of the species analyzed including control samples, and a data table of previously reported catalytic activities on LaNiO3 nanoscale systems (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Lei Wang: 0000-0002-6348-8344 Stanislaus S. Wong: 0000-0001-7351-0739 Notes

The authors declare no competing financial interest. M


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