Article pubs.acs.org/cm
Defining Crystalline/Amorphous Phases of Nanoparticles through X‑ray Absorption Spectroscopy and X‑ray Diffraction: The Case of Nickel Phosphide Liane M. Moreau,†,‡ Don-Hyung Ha,†,‡ Haitao Zhang,‡ Robert Hovden,§ David A. Muller,§,⊥ and Richard D. Robinson*,‡ ‡
Department of Materials Science and Engineering and §School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States ⊥ Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, United States S Supporting Information *
ABSTRACT: In this study we elucidate the structural distinctions between amorphous and crystalline Ni2P nanoparticles synthesized using tri-n-octylphosphine (TOP), through X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and inductively coupled plasma (ICP). We determine the differences in their chemical and atomic structure, which have not been previously reported, yet are essential for understanding their potential as nanocatalysts. These structural characteristics are related to the corresponding nanoparticle magnetic properties analyzed by performing magnetic measurements. XAS results reveal a significant P concentration in the amorphous nanoparticle sample − placing the stoichiometry close to Ni2P − despite XRD results that show only fcc Ni contributions. By comparing the long-range structural order from XRD to the short-range radial structure from EXAFS we conclude that both techniques are necessary to obtain a complete structural picture of amorphous and crystalline nanoparticle phases due to the limitations of XRD amorphous characterization. We find that phases are amorphous with respect to XRD when their offsets (deviations) from bulk interatomic distances have a standard deviation as high as ∼4.82. Phases with lower standard deviation (e.g., ≲1.22), however, are detectable as crystalline through XRD. The possible presence of amorphous phases should be considered when using XRD alone for nanoparticle characterization. This is particularly important when highly reactive reagents such as TOP are used in synthesis. By characterizing amorphous nickel phosphide nanoparticles that have a comparable stoichiometry to Ni2P, we confirm that TOP serves as a highly effective phosphorus source, even at temperatures as low as 230 °C. Unintended amorphous structure domains may significantly affect nanoparticle properties, and in turn, their functionality. KEYWORDS: EXAFS, nanoparticles, nickel phosphide, amorphous, crystalline
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INTRODUCTION Crystallinity control in nanoparticles holds the key to optimizing their functionality. It has been shown that crystallinity modification can lead to dramatic changes in nanoparticle properties.1−4 Although nanoparticles with controlled crystalline and amorphous characteristics are desirable in applications,2,4−6 the characterization of nanoparticle crystallinity is not well-established or defined. Nickel phosphides are under increasing interest as catalysts for hydrotreating in fuels.7,8 Ni2P has been reported to outperform commercially used catalysts in terms of resistance to poisoning.9−11 Several new studies have reported synthetic methods for nickel phosphide nanoparticles but little is known about their chemical and structural attributes. In recent years, tri-n-octylphosphine (TOP) has been used in the synthesis of metal phosphide nanoparticles through thermal decomposition of metal-phosphine complexes in the standard nanoparticle synthesis method.12 Brock et al.,13 Hyeon et al.,14 Schaak et al.,15 Tracy et al.,16 and Chiang et al.17 have used this method to © XXXX American Chemical Society
synthesize Ni and Ni−P nanoparticles. For the Ni nanoparticles, TOP was used as a ligand and thought to act only as a stabilizing surfactant on the surface; the particles were characterized, based on XRD, as pure fcc nickel.18 In a previous study, we discovered unintended phosphorus doping from these TOP ligands during the nickel nanoparticle synthesis by correlating X-ray absorption spectroscopy (XAS), a technique sensitive to amorphous structure, with traditional characterization tools (e.g., XRD and ICP).19 The phosphorus doping was high (∼6 atomic %) and has significant implications on the structural, magnetic, and catalytic properties of the nanoparticles.19 Amorphous nickel phosphide nanoparticles have been synthesized in literature,13,16,20 yet little is known about their chemical composition, local ordering, and the implications of their structure on their novel properties. These prior studies Received: October 29, 2012 Revised: May 3, 2013
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dx.doi.org/10.1021/cm303490y | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
nanoparticle structure. A thorough knowledge of the amorphous and crystalline phases within nanoparticles, as well as their relative contributions and effect on resulting structurally based properties, will enable optimization of nanoparticles for use in applications such as catalysis.
reveal the necessity to combine advanced XAS techniques with conventional techniques (e.g., XRD and TEM) to provide a complete structural and chemical characterization of the Ni−P nanoparticles that contain both crystalline and amorphous phases. Although techniques that enable the structural characterization of bulk material have been well-developed, the characterization of material at the nanoscale still presents significant challenges. The dominance of surface facets, small size, and poor crystalline order makes characterization of nanoparticles difficult. X-ray absorption spectroscopy (XAS) is a method that has been used to resolve the structure of colloidal nanocrystals.21−28 With the use of the high energy beam available through synchrotron radiation, XAS spectra can be obtained and used to gain insight into material structural properties.29,30 The structure of both XRD-detectable and XRD-amorphous materials can be determined through analysis of both the lower-energy XANES (X-ray absorption near edge) portion of the spectrum to resolve geometric structure, and the higher-energy quasi-periodic EXAFS (extended X-ray absorption fine structure) modulations to resolve radial structure, including interatomic distances, coordination numbers, and mean-squared disorder.31 By using XAS in combination with the conventional characterization techniques of XRD to determine crystalline behavior (long-range order) and TEM to determine size, morphology, long-range order, and phase, with ICP to confirm stoichiometry, and by performing magnetic measurements using a superconducting quantum interference device (SQUID) magnetometer to measure magnetic properties, structure−property relationships of the particles can be resolved. EXAFS and XRD complement each other by providing insight into short-range and long-range (crystalline) order, respectively. Through this investigation, we synthesize nickel phosphide nanoparticles using precursors with identical Ni/P ratios but with different reaction conditions. One reaction results in an amorphous phase and the other in a crystalline Ni2P phase. Although both phases have been previously reported in literature,13 their structural characteristics have not been described. For these reactions, we (1) report the atomic structure of amorphous and crystalline Ni2P nanoparticle samples (referred to hereafter as nanoparticle reaction products I and II, respectively), and particularly the differences in structure despite their similar elemental composition, (2) reveal the tendency for Ni atoms in nanoparticles to retain fcc positions even with a phosphorus-rich composition, (3) show how nanoparticle magnetization is highly dependent on the structure of the nanoparticle samples, (4) confirm that TOP can serve as a P source that is effective enough to produce nanoparticles with the stoichiometry of Ni2P even at temperatures as low as 230 °C, and (5) define the amorphous-limit of XRD in characterizing noncrystalline phases by comparing the XRD structural characterizations with EXAFS analysis. In conjunction with our previous Co−P32 and Ni19 nanoparticle studies, we propose that a phase is crystalline through XRD (XRD crystalline) when the standard deviation of offsets from bulk interatomic distances is minimal (e.g.,
