Effect of Surface Properties on the Microstructure, Thermal, and

Article pubs.acs.org/cm

Effect of Surface Properties on the Microstructure, Thermal, and Colloidal Stability of VB2 Nanoparticles Bürgehan Terlan,*,† Aleksandr A. Levin,‡ Felix Börrnert,§,#,⊥ Frank Simon,∇ Martin Oschatz,∥ Marcus Schmidt,‡ Raul Cardoso-Gil,‡ Tommy Lorenz,† Igor A. Baburin,† Jan-Ole Joswig,† and Alexander Eychmüller† †

Physical Chemistry, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany § IFW Dresden, PF 270116, 01171 Dresden, Germany # Speziallabor Triebenberg, TU Dresden, 01062 Dresden, Germany ⊥ Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom ∇ Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany ∥ Department of Inorganic Chemistry, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: Recent years have seen an increasing research effort focused on nanoscaling of metal borides, a class of compounds characterized by a variety of crystal structures and bonding interactions. Despite being subject to an increasing number of studies in the application field, comprehensive studies of the size-dependent structural changes of metal borides are limited. In this work, size-dependent microstructural analysis of the VB2 nanocrystals prepared by means of a size-controlled colloidal solution synthesis is carried out using X-ray powder diffraction. The contributions of crystallite size and strain to X-ray line broadening is separated by introducing a modified Williamson−Hall method taking into account different reflection profile shapes. For average crystallite sizes smaller than ca. 20 nm, a remarkable increase of lattice strain is observed together with a significant contraction of the hexagonal lattice decreasing primarily the cell parameter c. Exemplary density-functional theory calculations support this trend. The size-dependent lattice contraction of VB2 nanoparticles is associated with the decrease of the interatomic boron distances along the c-axis. The larger fraction of constituent atoms at the surface is formed by boron atoms. Accordingly, lattice contraction is considered to be a surface effect. The anisotropy of the sizedependent lattice contraction in VB2 nanocrystals is in line with the higher compressibility of its macroscopic bulk structure along the c-axis revealed by theoretical calculations of the respective elastic properties. Transmission electron microscopy indicates that the VB2 nanocrystals are embedded in an amorphous matrix. X-ray photoelectron spectroscopy analysis reveals that this matrix is mainly composed of boric acid, boron oxides, and vanadium oxides. VB2 nanocrystals coated with these oxygen containing amorphous species are stable up to 789 °C as evidenced by thermal analysis and temperature dependent X-ray diffraction measurements carried out under Ar atmosphere. Electrokinetic measurement indicates that the aqueous suspension of VB2 nanoparticles with hydroxyl groups on the surface region has a good stability at neutral and basic pH arising from electrostatic stabilization

1. INTRODUCTION Transition metal (TM) diborides are characterized by a very high melting temperature, hardness, chemical stability, and good electrical conductivity. Recently, improved performance characteristics are reported in application fields including both energy storage and catalysis, as a result of scaling down these materials to the nanoregime.1−4 Investigations on the size dependent structural characterization of metal borides are relatively fewer compared to other classes of refractory compounds, for example, nanoscaled metal oxides. The relatively small amount of publications in the field is mainly © XXXX American Chemical Society

due to limitation of conventional synthesis methods which do not allow an effective control of the size and the size distribution of nanoscaled TM diborides. Metal borides are generally obtained at very high temperatures (T > 1100 °C)5 which are not suitable for the isolation of metastable nanostructured systems. The use of low temperature colloidal synthesis in aqueous or organic media leads to nanoscaled Received: May 18, 2015 Revised: July 7, 2015

A

DOI: 10.1021/acs.chemmater.5b01856 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

hexagonal structure, boron atoms form two-dimensional honeycomb sheets separated by hexagonal metal layers. Investigations on electron density distribution in VB2 based on electron density reconstructed from single-crystal XRD as well as derived from theoretical calculations reveal a model including both B−B as well as B−V bonding interactions.37,38 However, a comprehensive analysis of the size-dependent changes of the structure has not yet been reported. In the present work, a more general interpretation based on a quantitative analysis of the microstructure using powder XRD line profile analysis is targeted. For the analysis of “crystallite size” and “lattice strain” broadening, a modified Williamson− Hall method is introduced which considers different convolution procedures depending on true reflection profile shapes. The structural changes with respect to the macroscopic VB2 were investigated and compared with the results obtained from quantum chemical calculations. Furthermore, the samples were characterized using transmission electron microscopy (TEM), Brunauer−Emmett−Teller (BET) analysis based on the physisorption of nitrogen, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared (ATR−FTIR) spectroscopy, and differential thermal analysis (DTA)/thermogravimety (TG). In addition, the colloidal stability of the nanoparticles in aqueous suspension is examined using sedimentation tests as well as electrokinetic measurements.

metal borides but at the expense of crystallinity and stability.6−15 Additional synthetic approaches including mechanochemical synthesis via ball milling and solid state reactions under autogenous pressure have to be considered with care due to difficulty of controlling particle size, contamination problems, and cost issues.16 In general, very hard materials such as metal borides demand very high energy input for the particle size reduction. According to recently developed synthetic methods including mechanochemical synthesis with a reductive assistant17 and inorganic molten salt technique,18 nanoparticles of TM diborides can be prepared at relatively mild temperatures allowing a much better control of purity, crystallinity, and nanoparticle size. An efficient method for characterizing the microstructure of nanomaterials is the powder X-ray diffraction (XRD) line profile analysis which allows separating the contributions of crystallite size and lattice strain to line broadening. Hence, a more accurate estimation of the crystallite size could be obtained compared to the standard Scherrer analysis19 which neglects the strain contribution. XRD line profile analysis offers some benefits compared to electron microscopy which is the common method of choice for the microstructural characterization of nanomaterials. First of all, it provides the average size of single crystallites, i.e., the size of coherent scattering domains with very high statistical relevance, whereas the particles (grains) observed by electron microscopy may contain one or more crystallites. Moreover, XRD allows nondestructive data collection with excellent resolution in the reciprocal space. XRD line profile analysis of nanomaterials was subject to a variety of studies including nanostructured Ge,20,21 CeO2,22 TiO2,23,24 and hydroxyapatite.25 In these studies, the microstructure is analyzed by assuming the entire X-ray reflection profiles only as purely Lorentzian (Cauchy approach). Shen et al.26 studied the residual strain in ferromagnetic Fe80Cu20 by taking the difference between Cauchy or Gaussian type line broadening into consideration. In principle, these approaches allow only a rough estimation of crystallite size and lattice strain due to neglect of true profile type which is more likely not pure Lorentzian or Gaussian but a pseudo-Voigt one, i.e., a mixture of Lorentzian and Gaussian contributions. A line profile analysis was carried out for CuTe and PbTe27 nanoparticles using strain-size broadening relation for pseudo-Voigt reflections. However, in this study, a very general description of correction for instrumental broadening is considered. A distinct classification between different reflection profile shapes is missing. Lattice strain is an important microstructural quantity which directly influences optical, electronic, and mechanical properties of nanocrystals.28−32 In general, nanocrystals can tolerate much higher strain than their macroscopic counterparts.33,34 This microstructural flexibility is explained by the distribution of lattice strain over the large fraction of under-coordinated surface atoms whereas bulk materials rather form strain-relaxing crystalline defects. Theoretical investigations reveal straininduced structural stiffening of most nanostructured materials.33,35 Gilbert et al.30 and Gu et al.36 have observed straininduced stiffening of nanocrystals by determining size-dependent vibration frequency from temperature-dependent extended X-ray absorption fine structure mesurements. This correlation makes lattice strain a potentially useful design parameter in terms of mechanical properties. Vanadium diboride VB2 is one of the transition metal diborides with AlB2 (C32) structure type. In the corresponding

2. EXPERIMENTAL PROCEDURES 2.1. Synthesis. VB2 nanocrystals were obtained by a colloidal synthesis route as described by Portehault et al.18 for the preparation of several other metal borides. VB2 nanocrystals were isolated from the eutectic mixture of anhydrous LiCl (Sigma-Aldrich, ≥98.0%) and NaCl (Alfa Aesar, 99.99%) when VCl3 (Alfa Aesar, 99%) and NaBH4 (Sigma-Aldrich, 99.99%) were used as starting materials. For a typical synthesis, the metal chloride and NaBH4 used as reducing agent (see Supporting Information, section S1) were thoroughly mixed with 2.5 g of the salt mixture LiCl:NaCl (65−35 wt %) under inert atmosphere in an argon-filled glovebox. The amounts of metal source and the salt mixture were kept constant while the boron content was varied (V/B = 1:2−8). A corundum crucible was used as reaction container which was placed in a quartz ampule. The reaction was carried out at 900 °C for 4 h under argon atmosphere (Praxair, >99.999%, purified with Air Liquide Oxisorb catalyst). The melt was cooled during 4 h from 900 °C to ambient temperature. Black powders were recovered by washing several times with water and acetone in order to remove excess of LiCl/NaCl salt and dried at room temperature overnight. All samples were stored in air under room temperature prior to further analyses. 2.2. XRD Investigation. The crystal structure of the samples was characterized using an X’Pert PRO diffractometer (PANalytical B.V., Netherlands) designed in focusing Bragg−Brentano geometry and supplied with a solid state X’Celerator linear detector. Cu Kα1 radiation, λ = 1.540598 Å, monochromatized by a primary Ge(111) monochromator of Johansson type was used. The XRD measurements were carried out utilizing symmetrical θ−2θ scan mode. The XRD patterns were corrected to Δ2θzero shift using additional measurements of the powders mixed with Si 640c NIST powder standard.39 The unit cell parameters of the hexagonal VB2 phase were calculated from the indexed reflections of this phase by means of the program CelSiz.40 All reflections of the VB2 phase were taken into account for the calculations. The crystallite size and lattice strain of the VB2 samples were investigated by means of X-ray line broadening analysis. The strain-size analysis was carried out using the program SIZECR41 by means of a modified Williamson−Hall method after an appropriate correction of the reflections for the instrumental broadening according to Warren42 and Rehani et al.43 depending on the XRD reflection profile type (Gaussian, Lorentzian, or pseudo-Voigt). B


Article

Chemistry of Materials The crystal size broadening is expressed through the Scherrer formula:19 FWHM size = K · λ /(D·cos(θB))

heating rate of 10 K/min and for the subsequent temperature ramps up to 900 °C with a heating rate of 5 K/min. 2.3. Electron Microscopy Investigation. TEM, high resolution TEM (HRTEM), and selected area electron diffraction (SAED) investigations were performed on a JEOL JEM-2010F transmission electron microscope retrofitted with two CEOS third-order spherical correctors. The electron acceleration voltage was set to 80 kV to minimize knock-on damage. Samples were prepared by evaporating a drop of diluted suspension in a water/ethanol mixture on a carboncoated copper grid. The analyses of the transmission electron micrographs were performed with the Digital Micrograph software (Gatan Inc.). 2.4. XPS Investigations. All XPS studies were carried out by means of an Axis Ultra photoelectron spectrometer (Kratos Analytical, Manchester, U.K.). The spectrometer was equipped with a monochromatic Al Kα (hν = 1486.6 eV) X-ray source of 300 W at 15 kV. The kinetic energy of photoelectrons was determined with a hemispheric analyzer set to pass energy of 160 eV for wide-scan spectra and 20 eV for high-resolution spectra. During all measurements, electrostatic charging of the sample was avoided by means of a low-energy electron source working in combination with a magnetic immersion lens. Later, all recorded peaks were shifted by the same value to set the C 1s peak to 285.00 eV. Quantitative elemental compositions were determined from peak areas using experimentally determined sensitivity factors and the spectrometer transmission function. The spectrum background was subtracted according to Shirley.50 The high-resolution spectra were deconvoluted by means of the Kratos spectra deconvolution software. Free parameters of component peaks were their binding energy (BE), height, full width at half-maximum, and the Gaussian−Lorentzian ratio. 2.5. N2 Physisorption Analysis. The studies were carried out at −196 °C on an Autosorp 1C apparatus (Quantachrome Instruments, Boynton Beach, U.S.A.). The BET surface area was calculated from the isotherms in the relative pressure range P/P0 = 0.05−0.2). The particle size was calculated as DBET = 6000/(ρ·Sw), where ρ is mass density of VB2 and Sw is the measured surface area of VB2 sample. 2.6. Vibrational Spectroscopy. ATR−FTIR spectra were recorded on a Thermo Scientific Nicolet 8700 spectrometer by contacting the smart iTR diamond plate on a dry sample. The spectra were collected for 16 scans at a resolution of 2 cm−1 in the range of 4000−650 cm−1. 2.7. Thermal Analysis. DTA/TG measurements were performed using a Netzsch STA 449 C analyzer. The samples were placed in Al2O3 crucibles and heated under Ar atmosphere with a heating rate of 10 K/min from room temperature to 1500 °C. 2.8. Electrokinetic Measurements. Beckman Coulter Delsa Nano C instrument was used for the determination of the zeta potential of nanocrystals in aqueous suspension at 25 °C. The pH of nanoparticle dispersions was adjusted by adding small portions of 0.1 mol/L HCl or 0.1 mol/L NaOH. The measurement was repeated three times, and the mean values were reported as the final result. 2.9. Theoretical Calculations. For the theoretical considerations, the density-functional theory (DFT) was employed with the localdensity approximation (LDA) as implemented in the SIESTA method for ab inito order-N materials simulation (Troullier-Martins pseudopotentials, DZP basis set).51

(1)

where K = 0.94 if FWHM (full width at half-maximum of the reflection) is used,44 λ is the wavelength of the X-ray radiation used, D is the crystallite size, and θB is the central-Bragg angle. Strain induced broadening arising from lattice imperfections and distortions is calculated using the Stokes−Wilson expression for microstrain broadening:45

FWHM strain = K strain· s · tan(θB)

(2)

where s is strain and Kstrain = 4 for microstrain in crystallites. To separate the contribution of both strain and size effects to the FWHM, corrected to instrumental broadening (FWHMcorr), a Cauchy−Cauchy model or the so-called Williamson−Hall method46 is used for the reflections of Lorentzian profile type:

FWHMcorr = FWHM size + FWHM strain

(3)

Applying both the Scherrer relation and the Stokes−Wilson expression for size and strain calculations to eq 3 yields

FWHMcorr ·cos(θB) = K · λ /D + s ·K strain· sin(θB)

(4)

Accordingly, the strain s and the size D of the crystallites can be extracted from the slope and y-intersect of the relationship when y = FWHMcorr·cos(θB) is plotted with respect to x = Kstrain·sin(θB). However, the underlying assumption of the Williamson−Hall analysis is that all the constituent reflection profiles are of purely Lorentziantype, which will rarely be valid in practice.47 In case of a Gaussian-type reflection broadening, the Gaussian−Gaussian approximation is valid:42

FWHMcorr 2 = FWHM size 2 + FWHM strain 2

(5)

For pseudo-Voigt components the respective relation takes the form48

FWHMcorr = FWHM size + (FWHM strain 2/FWHM)

(6)

Replacing the expressions for size and strain effects (eqs 1 and 2) in eqs 5 and 6 and rearranging yields the following equations for Gaussian (eq 7) and pseudo-Voigt (eq 8) type of line broadening, respectively: FWHMcorr 2·cos2(θB) = (K · λ /D)2 + s 2 · K strain 2·sin 2(θB)

(7)

and FWHMcorr ·cos(θB) = K ·λ /D + s 2 · (K strain· sin(θB))2 /(FWHMcorr ·cos(θB))

(8)

where size D and strain s can be calculated from the y-intersect and the slope of the linear relationships y = (K · λ/D)2 + s2·x and y = (K·λ/D) + s2·x for Gaussian and pseudo-Voigt type of line broadening, respectively. XRD reflections were approximated as Lorentzian, Gaussian, or pseudo-Voigt profiles in dependence on the experimental value of the ratio FWHMobs/β in comparison to the limit values of the Lorentzian, Gaussian, or pseudo-Voigt reflection profiles where FWHMobs and β are the observed FWHM and the integral breadth of the reflection, respectively.49 For assignment of the thermal effects observed on the DTA/TG analysis XRD experiments at high temperature were performed on a STOE-StadiP-MP (Stoe & Cie, Germany) powder diffractometer (Debye−Scherrer geometry, Stoe-linear-PSD detector, Cu Kα1 radiation, λ = 1.540598 Å, curved Ge(111) monochromator) equipped with a high temperature attachment. A dehydrated sample (preheated up to 500 °C in a DTA equipment) was filled in a quartz capillary (0.5 mm diameter) and sealed under argon atmosphere. The temperature program considered the heating of the sample up to 500 °C with a

3. RESULTS AND DISCUSSION The XRD patterns shown in Figure 1 mainly consist of the characteristic reflections of VB2 (space group P6/mmm and unit cell parameters a = 2.9972(2) Å and c = 3.0560(3) Å for the macroscopic single crystal37). Additional weak reflections become apparent when excess boron precursor (V/B ≥ 1:5) was used. These reflections are most likely attributed to other vanadium borides and/or oxidized vanadium−boron species. The synthesis of metal borides in the inorganic molten salt allows an effective size tuning of the nanoparticles by adjusting C


Article


Figure 2. Williamson−Hall plot of a VB2 sample obtained at 900 °C with VCl3/NaBH4 = 1:2 with lattice strain according to eq 8. Figure 1. XRD patterns of VB2 nanocrystals obtained at 900 °C with VCl3/NaBH4 = 1:2, 1:5, and 1:8. The XRD patterns have been shifted vertically for better visualization. Miller indices hkil of the VB2 reflections are indicated. Reflections denoted by an asterisk belong to impurity phases. In the inset the XRD pattern in the vicinity of the 1011̅ reflection is shown.

Hall analysis of other VB2 samples. Possible explanations for these deviations are the experimental error and/or the insufficient flexibility of the current model which does not account for anisotropic variation of lattice strain.20,21,26,52 Strongly overlapped reflections at higher diffraction angles as well as reflections overlapped with those of impurities were excluded from size and strain calculation. After exclusion of most rejected outliers, the calculations revealed that the broadening of the reflections is due to both size and strain effects as the experimental data indicated clearly a positive slope for all samples investigated (see Supporting Information, Figure S3). Figure 3 shows the size dependent variation of unit cell parameters a and c, unit cell volume V, and lattice strain s of VB2 nanocrystals. In general, the unit cell volume of

the boron-source concentration.18 This is rationalized in the framework of classical crystal growth theory as a nucleationdriven phenomenon. The inset of Figure 1 shows the most intense (hkil = 1011̅ ) reflections of VB2 nanocrystals prepared using the initial ratios of V/B = 1:2, 1:5, and 1:8, respectively. The reflections become broader with increasing NaBH4 concentration. In addition, their positions are shifted to higher diffraction angles indicating a lattice contraction. The average crystallite size of as-synthesized VB2 nanocrystals with V/B = 1:8 is D ≈ 10 nm in good agreement with the estimated diameter of ∼11 nm according to BET analysis based on N2 sorption (see Supporting Information, section S2). The specific surface area values according to the BET method and XRD (under the assumption of spherical particles with a diameter of 9.9(1.4) nm) are 111 m2/g and 120(26) m2/g, respectively. This consistency provides evidence that formation of dense agglomerates of VB2 nanocrystallites is not favored. Such a high surface area could provide benefits, for example, for energy storage applications by improving the discharge efficiency of VB2 anodes in aqueous batteries.1−3 The form of the X-ray reflection profiles of the VB2 samples are mostly pseudo-Voigt (0.637 < FWHMobs/β < 0.939). Therefore, the crystallite size D and the lattice strain s are estimated from the slope and y-intersect of the linearly fitted data obtained by plotting the term FWHMcorr·cos(θB) with respect to (Kstrain·sin(θB))2/(FWHMcorr·cos(θB)) according to eq 8 using the strain-size broadening relation for pseudo-Voigt reflections (see Supporting Information, section S3). Indeed, a pseudo-Voigt model gave a better fit of the experimental data with smaller uncertainty when compared with calculations using Cauchy−Cauchy and Gaussian−Gaussian approximations for the same set of reflections. A representative Williamson− Hall plot of VB2 nanocrystals prepared using the initial ratio of V/B = 1:2 is shown in Figure 2. The data indicates clearly a positive slope. Accordingly, the broadening of the reflections is due to both size and strain effects. Apparently, the data does not entirely obey the present formulation of lattice strain as evidenced by the scattering of the points away from a linear expression. Similar deviations were observed by Williamson−

Figure 3. Variation of unit cell volume Vunitcell, lattice strain s (a), and unit cell parameters a and c (b) vs crystallite size D for VB2 nanocrystals. For comparison, the equilibrium values a0, c0, and V0 of the macroscopic VB2 single crystal taken from Terlan37 are indicated as horizontal lines. The crystal structure of VB2 is shown in the inset in (b). D


Article


crystal lattice parameters. Generally, a ring pattern indicates that the size of the crystals is substantially smaller than the effective aperture size. In order to assess the crystal size, a dark field imaging was performed where a small section from the diffraction ring in the back focal plane is selected and, thus, only a small fraction of crystallites having a corresponding orientation was imaged. This was repeated for several different orientations. Figure 4b shows a representative dark field image. The largest VB2 crystals for the initial ratios of V/B = 1:8 observed were about 20 nm in size, but the majority were below 10 nm. This is in agreement with the XRD and N 2 physisorption results. VB2 nanocrystals of larger sizes (initial V/B = 1:2 and 1:5) are also embedded in an amorphous matrix (see Supporting Information, section S5). Unfortunately, this matrix prevents the interpretation of the microstructure, particularly for lattice strain; regardless, the imaging with spherical aberration corrected electron optics. Additionally, since boron is a very light element, the sample and especially the amorphous matrix are very susceptible to the so-called knock-on beam damage. Therefore, low electron energy was used for imaging to reduce this type of beam damage. Nevertheless, the electron beam induced a substantial sample drift due to radiolysis type beam damage in the amorphous matrix hindering high quality image recording. An example of the achievable image quality is shown in Figure 4c. Generally, the assessment of lattice strain in surfaces and interfaces by TEM is extremely demanding. Not only have the imaging artifacts introduced by the electron optics have to be considered but also the multiple interactions of the imaging electrons inside the sample have to be considered.56 This can only be done with excellent micrographs with high signal-tonoise ratio and highly oriented samples as well as matching imaging simulations. The presence of an amorphous matrix as a coating was also indicated in the case of several metal boride nanoparticles prepared by chemical reduction of the respective metal salts with NaBH4.18,57 It has been claimed that this amorphous compound consists of oxidized boron species which protect the nanocrystalline core from growth. To characterize the concentrations and the oxidation states of the elements at the surface region of nanosized VB2, XPS measurements were carried out. Figure 5a shows a typical XPS survey (wide scan) spectrum of a VB2 sample. The presence of oxygen indicates that the surface of the VB2 particles is partly or fully oxidized. Besides vanadium and boron, the wide-scan spectra also showed the presence of carbon, nitrogen, and oxygen. In addition, several auger signals were detected. The O KLL represents the oxygen Auger series with the Auger peaks O KL23L23 at ca. 978 eV, O KL1L23 at ca. 1000 eV, and O K1L1 at ca. 1015 eV, whereas the vanadium Auger series consisting of the V L3M45M45 peak at ca. 977 eV, V L3M23M45 peak at ca. 1015 eV, and V L23M23M23 peak at ca. 1050 eV are assigned to V LMM. Figure 5 also shows the high resolution B 1s, V 2p and, O 1s spectra of the surface of nanoscopic VB2. The B 1s spectrum was deconvoluted into four component peaks A, B, C, and D (the parameters of the component peaks are given in Table 1). Component peak A (at BE of 193.2 eV) is attributed to B2O3.58 Near room temperature B2O3 undergoes several intermediate reactions with water vapor and forms B(OH)3. Therefore, boron oxides carrying OH groups also contributed to component peak A.59 The BE found for component peak C (188.3 eV) is typical for VB2.60 Component peak D (187.3 eV) resulted from nonoxidized boron B0, which was also detected

nanocrystals with an average crystallite size of larger than ca. 20 nm is close to the equilibrium value V0 obtained from XRD single crystal measurements of macroscopic VB2.37 Below that size regime, a remarkable increase of lattice strain is observed together with a significant decrease of the unit cell volume. Taking the bulk VB2 as a reference, the highest unit cell volume contraction is found to be 0.47%. The data suggest that lattice contraction for average crystallite sizes smaller than ∼20 nm is accompanied by decrease of the cell parameter c whereas a significant deviation from the equilibrium value a0 is not observed for the cell parameter a (Figure 3b). Different tendencies of cell parameters in case of lattice contraction were also observed for tetragonal Sn,53 rhombohedral Bi,53 and TiO254 nanoparticles. This result reflects the anisotropy of the structure. In order to check whether compositional changes are influencing the lattice parameters, structure refinements of powder X-ray data based on the Rietveld method were performed for VB2 nanocrystals with starting compositions V/B = 1:2, 1:5, and 1:8 (see Supporting Information, section S4). Compositional deviations from the ideal stoichiometry are generally associated with cation deficiency, in case of metal diborides.55 The refinement of the occupancy for the vanadium position did not indicate any relevant deviation from the target value. Accordingly, VB2 nanocrystals prefer to be strained rather than forming strain-relaxing defects. For a better understanding of the sample’s microstructure and morphology a real space imaging method is applied, i.e., TEM. The powder grains show sizes of about 0.5 μm and consist of nanocrystals embedded in an amorphous matrix. SAED records were taken with the smallest available aperture having an effective diameter of 100 nm (see the aperture shadow in Figure 4a). They resulted in ring patterns due to the randomly oriented nanosized crystals and a diffuse ring originating from the amorphous matrix as shown in the inset of Figure 4a. The rings could be indexed with the respective

Figure 4. (a) TEM micrograph of VB2 nanocrystals obtained at 900 °C with VCl3/NaBH4 = 1:8. Corresponding SAED pattern is shown in the inset. (b) Dark field TEM and (c) HRTEM micrograph of VB2 nanocrystals. The hkl indices of the rings are given in (a) and (c). E


Article


Figure 5. Survey (a) and high resolution B 1s (b), O 1s, and V 2p (c) XPS spectra recorded from nanoscopic VB2 obtained at 900 °C with VCl3/ NaBH4 = 1:8.

Table 1. Fitting Results of High Resolution XPS Spectra of V, B, and O Species at the Surface Region of VB2 Nanocrystals Prepared Using Excess Boron Precursor (V/B = 1:8) peak

BE (eV)

FWHM (eV)

atom %

assigned species

B 1s A B 1s B B 1s C B 1s D V 2p3/2 F V 2p3/2 G O 1s H O 1s I

193.2 190.4 188.3 187.3 517.1 516.3 532.6 530.4

1.877 1.277 1.277 1.277 1.766 1.766 2.219 1.739

80.74 4.67 9.08 5.51 72.8 27.2 58.56 41.44

B2O3, B(OH)3 intermediate states VB2 B0 V2O5 VO2 B2O3, B(OH)3 VO2, V2O5

Figure 6. ATR−FTIR spectrum of VB2 sample obtained at 900 °C using excess boron precursor (VCl3/NaBH4 = 1:8). As reference compounds the spectra of B(OH)3 and V2O5 are also shown.

by Stuart et al. in the surface structure of VB2 nanoparticles prepared using ball-mill synthesis.61 The origin of component peak B, which arose at 190.4 eV, is not very clear. Its BE points to the presence of BN,62 but it could be also assumed that the intermediate state of vanadium boride and boron oxides are present in the surface region of the particles. Due to spin−orbit interaction (or coupling) the V 2p is composed of the V 2p3/2 and V 2p1/2 peaks (Figure 5c). In order to study the binding states of vanadium, the V 2p3/2 spectrum was deconvoluted into two component peaks F and G (the corresponding component peaks of the V 2p1/2 spectrum were denoted with F′ and G′). According to their BE values component peaks G (516.3 eV) and F (517.1 eV) present oxygen-containing vanadium compounds VO2 (V4+) and V2O5 (V5+), respectively.62 The V 2p3/2 component peak of VB2 which should be observed at 513.2 eV60 is not present in the spectrum. On the other hand, the B 1s spectra clearly showed the presence of VB2. The missing VB2 component peak is explained by the small fraction of VB2 compared to the oxides at the sample surface. The O 1s spectrum was also deconvoluted into two component peaks. Component peaks I at 530.4 eV results from VO2 (V4+) and V2O5 (V5+), whereas component peak H at 532.6 eV reveals the presence of B2O3 and B(OH)3.62 A comparison between the relative concentrations of vanadium, boron, and oxygen species obtained from the fitting results of high-resolution XPS spectra reveals that boron oxides are the most abundant species on the surface region of VB2 nanocrystals (Table 1). Considering the air-free synthesis conditions of VB2 nanocrystals the oxidation of the surface structure should result from the after-synthesis treatment conditions (cf. synthesis, section 2.1). To investigate the oxidation process of VB2 nanocrystals as a function of time, ATR−FTIR analysis is carried out. Figure 6 shows the ATR−FTIR spectra of as-synthesized VB2 nanocrystals (initial V/B = 1:8) and those exposed to ambient conditions (under air atmosphere, room temperature) for 1 month, 6 months, and 12 months. In the case of as-synthesized

nanocrystals the absence of any signal indicates that an immediate internal oxidation of VB2 nanocrystals after synthesis does not take place in air. After 1 month the characteristic peaks of V−O63 and B−O64,65 bond vibrations are observed. The broad peak at 3178 cm −1 belongs to the O−H bond stretching vibration. The peaks at 1190 and 1406 cm−1 are assigned to B−O−H in-plane bending and B−O stretching vibrations, respectively. The peak at 1003 cm−1 results from stretching vibration of the V−O bond. The band representing the bridging V−O−V stretching mode which should be present at 776 cm−1 appears to be buried below the broad band at 700 cm−1 attributed to B−O−H out-of-plane bending. The observed ATR−FTIR peaks grow in magnitude during the course of oxidation up to 12 months. In order to analyze the influence of the oxidation induced volume change on the microstructure of VB2 nanocrystals XRD line profile analysis is carried out for as-synthesized VB2 nanocrystals (initial V/B = 1:8) and compared with those exposed to highly humid atmospheric conditions for 1 week. Both the average crystallite size and lattice strain values did not indicate any significant difference between the as-synthesized and aged sample (see Supporting Information, section S6). Accordingly, it is unlikely that the observed increase of lattice strain in smaller VB2 nanocrystals is caused by the volume change induced by oxidation. Further insight into the experimental results is provided by DFT calculations carried out exemplarily for a single VB2 nanoparticle with a hexagonal-prismatic habit that was a cutout from the bulk material (see Supporting Information, section S7). The particle has a hexagonal base and nine layers of either vanadium or boron atoms; the exact stoichiometric composition is V79B210 (ratio V:B ≈ 1:2.76). Oxygen atoms are not included in the model considering the synthesis VB2 nanocrystals in air-free conditions (cf. synthesis, section 2.1). F


Article


unsaturated structures along the z-axis reveals that the decrease of B−B distance is much higher than that of the V−V distance. Since the nanoparticle surface, the dominant part of the structure, is mainly composed of boron atoms, this result supports the conclusion that the lattice contraction is a surface effect. A higher contraction of interatomic distances at the surface is also reported for Au nanocrystals using electron diffraction and molecular dynamics simulations and explained by decrease of coordination number.34 The thermal stability of VB2 nanocrystals (initial V/B = 1:8) was analyzed using DTA/TG (Figure 8). The TG analysis

Geometry optimization of this bulk cutout leads to changes at the surface due to unterminated valences of the surface atoms. A saturation scheme with hydrogen atoms was used to prevent these surface reconstruction processes. In both cases, averaging over boron−boron distances showed that the contraction along the c-axis is larger than along the a-axis (see Supporting Information, section S8). The trend is slightly reduced upon saturation, but still present. For better visualization, Figure 7

Figure 7. Energy−strain dependence of the VB2 nanoparticle (a) without and (b) with hydrogen saturation. The total energies were obtained by compressing and stretching the original bulk cutout independently either along the a- (black curves) or c-axis (red curves). The strain ε is equal to the ratio ΔL/L0 where ΔL and L0 represent the change of the lattice parameter after compressing or stretching and original lattice parameter, respectively. The data points were fitted with a cubic polynomial to consider anharmonic effects. The value of the optimized bulk (ε = 0) is marked by the vertical dashed lines.

Figure 8. DTA/TG data of VB2 nanocrystals obtained at 900 °C with VCl3/NaBH4 = 1:8. The onset of the oxidation process is marked by the arrow (789 °C).

shows the energy of the optimized structure as a function of contraction/elongation in either the a- or c-direction. The saturated nanoparticle has a slightly lower c lattice parameter (97.7% of the bulk value), whereas the lattice parameter a stays nearly constant (100.4% of the bulk value). The anisotropy of the size dependent lattice contraction in VB2 nanocrystals is in line with higher compressibility of its crystalline bulk structure along the c-axis revealed by calculations of the respective elastic properties. Table 2 shows the elastic constants Cij, bulk

indicates a weight loss of 8.9 wt % up to ∼460 °C in a two-step process. According to DTA, the decay of the relative mass is correlated with an endothermic peak starting at ca. 100 °C and originating from the dehydration of boric acid B(OH)3.70 Additional mass loss starting from ∼1100 °C indicates evaporation of B2O3. In addition, the data clearly show an exothermic peak at 820 °C (onset temperature at 789 °C). Possible explanations of this exothermic effect include crystallization, oxidation, or release of lattice strain in VB2 structure. For a better understanding of the origin of this exothermic peak temperature-dependent X-ray powder diffraction was performed (Figure 9). Up to 600 °C VB2 phase remains as the dominant phase in the XRD pattern. The most intense (hkil = 101̅1) reflections are shifted to lower diffraction angles with increasing temperature indicating the relaxation of the compressed lattice. The reflections of the VB2 phase become weaker at 800 °C and almost disappear at 900 °C. After cooling to room temperature, the XRD pattern mainly consists of reflections of VBO371and VO0.972 but VB2 is not present in the sample. This result supports the conclusion that the exothermic peak at 820 °C observed by the DTA study is associated with the internal oxidation of VB2 nanoparticles by the embedding matrix containing oxides of boron and vanadium species. The reflections of these compounds were not detected in the XRD patterns due to their amorphous nature. VB2 nanocrystals are stable up to 789 °C which is the onset temperature of the oxidation process. To study the effect of the surface elements on the stability of VB2 nanocrystals in aqueous suspension, electrokinetic measurements and sedimentation tests were carried out (Figure

Table 2. Calculated Elastic Constants (Cij), Young’s Modulus (E), and Bulk Modulus (B) of the Crystalline VB2 Bulk Structurea

a

C11

C12

C13

C33

C44

E[101̅0]

E[0001]

B

705.4

114.5

122.8

589.6

273.1

688.2

552.8

302.3

All values are given in GPa.

modulus B, and Young’s moduli E calculated using optimized unit-cell parameters. The calculated equilibrium unit-cell parameters for bulk VB2 (aopt = 2.9559 Å and copt = 2.9193 Å) are in reasonable agreement with experimental data37 within the usual limits of LDA. In general, the calculated elastic property values are close to published theoretical results.66−69 The anisotropy can be seen by the comparison of both C11 and C33 values and Young’s moduli along [101̅0] and [0001]. It is clear that the crystal is more compressible along [0001] rather than along [101̅0]. A widely accepted explanation of size-dependent lattice contraction of nanostructures is hydrostatic compression induced by interface and/or surface stress. A comparison between the interatomic distances of both saturated and G


Article


At high pH values (pH > 11.5) the compression of the electrical double layer is observed by decreasing the zetapotential values. By increasing the pH value, the ionic strength of the aqueous electrolyte solution is also increased. Consequently, the difference in concentration of the potential-determining ions in the electrochemical double layer and the bulk phase of the solution is decreased. VB 2 nanocrystals with hydroxyl groups on the surface region have a good stability in aqueous dispersions up to almost 1 week. This is attributed to the electrostatic stabilization as the zetapotential values determined at neutral pH are highly negative (z ∼ −50 mV). In general, a basic environment (z ∼ −50 mV) provides a better electrostatic stabilization for VB2 nanoparticles in comparison to acidic conditions (z ∼ 40 mV).

4. CONCLUSIONS A colloidal synthesis method in an inorganic salt mixture allowed size-controlled preparation of VB2 nanocrystals. The nanocrystals were investigated using a combination of complementary experimental (XRD, BET, TEM, XPS, ATR− FTIR, DTA/TG, electrokinetic measurements) and theoretical (DFT) methods. TEM analysis indicated that VB2 nanoparticles are embedded in an amorphous matrix which predominantly consists of boron oxide species according to XPS. Under ambient conditions this matrix slowly oxidizes, but an immediate internal oxidation of the nanocrystals does not take place after the synthesis. VB2 nanocrystals embedded in this partially or fully oxidized matrix have a good thermal stability up to 789 °C under Ar. Electrokinetic measurements revealed that VB2 nanocrystals have a good electrostatic stabilization in aqueous dispersions provided by the hydroxyl groups in the surface region. A size-dependent microstructural analysis of VB2 nanocrystals was performed using powder XRD. A modified Williamson−Hall method was introduced which considers true reflection profiles in the case of “crystallite size” and “lattice strain” reflection broadening. X-ray line profile analysis indicated that the reflection broadening of VB2 nanocrystals is due to both size and strain effects. For average crystallite sizes smaller than ca. 20 nm, the lattice strain becomes significant. In addition, a remarkable lattice contraction (in comparison to the values for the macroscopic VB2 single crystal) is observed in the same size regime. Both XRD and DFT calculations reveal that the lattice contraction is predominantly due to a decrease of the cell parameter c. DFT calculations support the conclusion that the lattice contraction of smaller VB2 nanocrystals is a surface effect. Size-dependent variations of the cell parameters are in line with elastic properties of the macroscopic VB2 and reflect the anisotropy of the structure. Uncovering microstructure property relationships for nanoscaled transition metal diborides is considered to be worthy of future studies in order to provide a better understanding in the application field.

Figure 9. XRD patterns of VB2 nanoparticles obtained at 900 °C using excess boron precursor (VCl3/NaBH4 = 1:8) measured in quartz capillary under Ar atmosphere at different temperatures. Bulk diffraction reflections for VB2 are indexed at the bottom, and the vertical dashed lines are given as guides to the eye for the respective reflections. Symbols: (open triangle) VBO3,71 (open circle) VO0.9.72 Reflections denoted with black squares could not be indexed to any known compound.

10). Positive zeta-potential values (z) determined in an acidic environment (pH < 4.4) indicate a positive net surface charge

Figure 10. Variation of zeta potential with pH value for VB2 nanocrystals obtained at 900 °C with VCl3/NaBH4 = 1:8. In the inset sedimentation tests of VB2 nanocrystals coated with boric acid dispersed in water are shown after 1 day (a), 1 week (b), 2 weeks (c), and 3 weeks (d) of sedimentation.

on the nanoparticle surface. B−OH groups covering the outer particle surface are protonated by the excess of H3O+ ions. With increasing the pH the degree of protonation is decreased and, consequently, the zeta-potential values are also decreased. The isoelectric point is found to be pH = 4.4, which is a typical value of an amphoteric surface. A further increase of pH leads to a progressive dissociation of the B−OH surface groups. Thereby, increasing the number of B−O− ions remaining on the nanoparticle surface increases the negative net surface charge, which results in an increase of the negative zeta-potential values. At pH ≈ 7 the zeta-potential remains constant. All B−OH groups, which are able to be dissociated, are present in their dissociated form (degree of dissociation = 1).

■

ASSOCIATED CONTENT

* Supporting Information S

The proposed reaction mechanism, Williamson−Hall plots, Rietveld refinements, N2 adsorption and desorption isotherms, and additional TEM micrographs of VB2 nanocrystals, XRD patterns of as-synthesized and aged samples, and schemes of VB2 nanocrystals as a cutout of the bulk material for theoretical calculations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.chemmater.5b01856. H


Article


■

(11) Li, H.; Yang, H.; Li, H. Highly active mesoporous Co-B amorphous alloy catalyst for cinnamaldehyde hydrogenation to cinnamyl alcohol. J. Catal. 2007, 251, 233−238. (12) Cheng, X.; Wu, B.; Yang, Y.; Xiang, H.; Li, Y. Fischer−Tropsch synthesis in polyethylene glycol with amorphous iron nanocatalysts prepared by chemical reduction in various solvents. J. Mol. Catal. A: Chem. 2010, 329, 103−109. (13) Kapfenberger, C.; Hofmann, K.; Albert, B. Room-temperature synthesis of metal borides. Solid State Sci. 2003, 5, 925−930. (14) Rades, S.; Kornowski, A.; Weller, H.; Albert, B. Wet-Chemical Synthesis of Nanoscale Iron Boride, XAFS Analysis and Crystallisation to α-FeB. ChemPhysChem 2011, 12, 1756−1760. (15) Rades, S.; Kraemer, S.; Seshadri, R.; Albert, B. Size and Crystallinity Dependence of Magnetism in Nanoscale Iron Boride, αFeB. Chem. Mater. 2014, 26, 1549−1552. (16) Carenco, S.; Portehault, D.; Boissière, C.; Mézailles, N.; Sanchez, C. Nanoscaled metal borides and phosphides: recent developments and perspectives. Chem. Rev. 2013, 113, 7981−8065. (17) Kim, J. W.; Shim, J. H.; Ahn, J. P.; Cho, Y. W.; Kim, J. H.; Oh, K. H. Mechanochemical synthesis and characterization of TiB2 and VB2 nanopowders. Mater. Lett. 2008, 62, 2461−2464. (18) Portehault, D.; Devi, S.; Beaunier, P.; Gervais, C.; Giordano, C.; Sanchez, C.; Antonietti, M. A General Solution Route toward Metal Boride Nanocrystals. Angew. Chem., Int. Ed. 2011, 50, 3262−3265. (19) Scherrer, P. Bestimmung der Grösse und der inneren Struktur von Kolloidteilchen mittels Röntgenstrahlen. Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, math-phys. Kl. 1918, 2, 98− 100. (20) Giri, P. K. Strain analysis on freestanding germanium nanocrystals. J. Phys. D: Appl. Phys. 2009, 42, 245402. (21) Kumari, S.; Singh, D. K.; Giri, P. K. Strain anisotropy in freestanding germanium nanoparticles synthesized by ball milling. J. Nanosci. Nanotechnol. 2009, 9, 5231−5236. (22) Zhou, X.-D.; Huebner, W. Size-induced lattice relaxation in CeO2 nanoparticles. Appl. Phys. Lett. 2001, 79, 3512. (23) Li, G.; Li, L.; Boerio-Goates, J.; Woodfield, B. F. High purity Anatase TiO2 Nanocrystals: Near Room-Temperature Synthesis, Grain Growth Kinetics, and Surface Hydration Chemistry. J. Am. Chem. Soc. 2005, 127, 8659−8666. (24) Santara, B.; Giri, P. K.; Imakita, K.; Fujii, M. Microscopic origin of lattice contraction and expansion in undoped rutile TiO 2 nanostructures. J. Phys. D: Appl. Phys. 2014, 47, 215302. (25) Venkateswarlu, K.; Chandra Bose, A.; Rameshbabu, N. X-ray peak broadening studies of nanocrystalline hydroxyapatite by Williamson-Hall analysis. Phys. B 2010, 405, 4256−4261. (26) Shen, T.; Schwarz, R.; Thompson. Soft magnetism in mechanically alloyed nanocrystalline materials. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 014431. (27) Jamwal, D.; Kaur, G.; Raizada, P.; Singh, P.; Pathak, D.; Thakur, P. Twin-Tail Surfactant Peculiarity in Superficial Fabrication of Semiconductor Quantum Dots: Toward Structural, Optical, and Electrical Features. J. Phys. Chem. C 2015, 119, 5062−5073. (28) Ouyang, G.; Yang, G.; Sun, C.; Zhu, W. Nanoporous Structure: Small is Stronger. Small 2008, 4, 1359−1362. (29) Smith, A. M.; Mohs, A. M.; Nie, S. Tuning the optical and electronic properties of colloidal nanocrystals by lattice strain. Nat. Nanotechnol. 2009, 4, 56−63. (30) Gilbert, B.; Huang, F.; Zhang, H.; Waychunas, G. A.; Banfield, J. F. Nanoparticles: Strained and Stiff. Science 2004, 305, 651−654. (31) Shahid, R.; Murugavel, S. Particle size dependent confinement and lattice strain effects in LiFePO4. Phys. Chem. Chem. Phys. 2013, 15, 18809−18814. (32) Rastei, M. V.; Heinrich, B.; Limot, L.; Ignatiev, P. A.; Stepanyuk, V. S.; Bruno, P.; Bucher, J. P. Size-Dependent Surface States of Strained Cobalt Nanoislands on Cu (111). Phys. Rev. Lett. 2007, 99, 246102. (33) Ouyang, G.; Zhu, W. G.; Sun, C. Q.; Zhu, Z. M.; Liao, S. Z. Atomistic origin of lattice strain on stiffness of nanoparticles. Phys. Chem. Chem. Phys. 2010, 12, 1543−1549.

AUTHOR INFORMATION

Corresponding Author

*(B.T.) E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dr. Paul Simon and Prof. Dr. Stefan Kaskel for helpful discussions and technical assistance. Financial support through Deutsche Forschungsgemeinschaft (DFG) M.ERANET project ICENAP and computational time by the Center for Information Services and High Performance Computing (ZIH) within the project QDSIM is gratefully acknowledged.

■

ABBREVIATIONS TEM, transmission electron diffraction; HRTEM, highresolution TEM; SAED, selected area electron diffraction; FTIR-ATR, Fourier transform infrared−attenuated total reflectance; DTA, differential thermal analysis; TG, thermogravimetry; TM, transition metals; XRD, X-ray diffraction; DFT, density functional theory; LDA, local-density approximation; BET, Brunauer−Emmett−Teller; BE, binding energy; FWHM, full width at half-maximum; XPS, X-ray photoelectron spectroscopy

■

REFERENCES

(1) Licht, S.; Wu, H.; Yu, X.; Wang, Y. Renewable highest capacity VB2/air energy storage. Chem. Commun. 2008, 3257−3259. (2) Licht, S.; Ghosh, S.; Wang, B.; Jiang, D.; Asercion, J.; Bergmann, H. Nanoparticle Facilitated Charge Transfer and Voltage of a High Capacity VB2 Anode. Electrochem. Solid-State Lett. 2011, 14, A83−A85. (3) Licht, S.; Hettige, C.; Lau, J.; Cubeta, U.; Wu, H.; Stuart, J.; Wang, B. Nano-VB2 Synthesis from Elemental Vanadium and Boron: Nano-VB2 Anode/Air Batteries. Electrochem. Solid-State Lett. 2012, 15, A12−A14. (4) Fan, X.; Xiao, X.; Chen, L.; Shao, J.; Zhang, L.; Li, S.; Ge, H.; Wang, Q. Superior Catalytic Effects of Transition Metal Boride Nanoparticles on the Revesible Hydrogen Storage Properties of LiMg-B-H System. Part. Part. Syst. Charact. 2014, 31, 195−200. (5) Greenwood, N. N.; Parish, R.; Thornton, P. Metal Borides. Q. Rev., Chem. Soc. 1966, 20, 441−464. (6) Wu, C.; Wu, F.; Bai, Y.; Yi, B.; Zhang, H. Cobalt boride catalysts for hydrogen generation from alkaline NaBH4 solution. Mater. Lett. 2005, 59, 1748−1751. (7) Taghavi, F.; Falamaki, C.; Shabanov, A.; Seyyedi, M.; Zare, M. An effective method for increasing the activity of nickel boride catalyst nanoparticles in hydrogenation reactions: Low-temperature hydrogen treatment. Appl. Catal., A 2013, 453, 334−340. (8) Zhu, Y.; Liu, F.; Ding, W.; Guo, X.; Chen, Y. Noncrystalline Metal-Boron Nanotubes: Synthesis, Characterization, and CatalyticHydrogenation Properties. Angew. Chem., Int. Ed. 2006, 45, 7211− 7214. (9) Zhu, Y.; Guo, X.; Shen, Y.; Mo, M.; Guo, X.; Ding, W.; Chen, Y. Preparation and catalytic property of a non-crystalline alloy of ironboron with one-dimensional nanostructures. Nanotechnology 2007, 18, 195601. (10) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjapanayis, G. C. Borohydride reductions of metal ions. A new understanding of the chemistry leading to nanoscale particles of metals, borides, and metal borates. Langmuir 1992, 8, 771−773. I


Article

Chemistry of Materials (34) Huang, W. J.; Sun, R.; Tao, J.; Menard, L. D.; Nuzzo, R. G.; Zuo, J. M. Coordination-dependent surface atomic contraction in nanocrystals revealed by coherent diffraction. Nat. Mater. 2008, 7, 308−313. (35) Ouyang, G.; Li, X. L.; Tan, X.; Yang, G. W. Size induced strain and stiffness of nanocrystals. Appl. Phys. Lett. 2006, 89, 031904. (36) Gu, Q. F.; Krauss, G.; Steurer, W.; Gramm, F.; Cervellino, A. Unexpected high stiffness of Ag and Au nanoparticles. Phys. Rev. Lett. 2008, 100, 045502. (37) Terlan, B. Experimental electron density reconstruction and analysis of titanium diboride and vanadium borides. Ph.D. thesis, TU Dresden, 2013. (38) Wagner, F. R.; Baranov, A. I.; Grin, Y.; Kohout, M. A PositionSpace View on Chemical Bonding in Metal Diborides with AlB2 Type of Crystal Structure. Z. Anorg. Allg. Chem. 2013, 639, 2025−2035. (39) Internation Tables for Crystallography; Wilson, A. J., Ed.; Kluwer Academic Publisher: Dordrecht/Boston/London, 1992; Vol. C: Mathematical, Physical and Chemical Tables, p 428. (40) Hay, D. CELSIZ Program; CSIRO Division of Materials Science and Technology: Clayton, Australia, 1995. (41) Levin, A. A.; Levichkova, M.; Hildebrandt, D.; Klisch, M.; Weiss, A.; Wynands, D.; Elschner, C.; Pfeiffer, M.; Leo, K.; Riede, M. Effect of film thickness, type of buffer layer, and substrate temperature on the morphology of dicyanovinyl-substituted sexithiophene films. Thin Solid Films 2012, 520, 2479−2487. (42) Warren, B. E. X-ray diffraction; Courier Dover Publications: 1990; Vol. 1, p 402. (43) Rehani, B. R.; Joshi, P. B.; Lad, K. N.; Pratap, A. Crystallite size estimation of elemental and composite silver nano-powders using XRD principles. Indian J. Pure Appl. Phys. 2006, 44, 157−161. (44) Langford, J. I.; Wilson, A. J. C. Scherrer after sixty years: A survey and some new results in the determination of crystallite size. J. Appl. Crystallogr. 1978, 11, 102−113. (45) Stokes, A. R.; Wilson, A. J. C. The diffraction of X-rays by distorted Crystal Aggregates-I. Proc. Phys. Soc. 1944, 56, 174−181. (46) Williamson, G. K.; Hall, W. H. X-ray Line Broadening from Filed Aluminium and Wolfram. Acta Metall. 1953, 1, 22−31. (47) Langford, J. L.; Delhez, R.; de Keijser, T. H.; Mittemeijer, E. J. Profile Analysis for Microcrystalline Properties by the Fourier and Other Methods. Aust. J. Phys. 1988, 41, 173−187. (48) Halder, N. C.; Wagner, C. N. J. Separation of particle size and lattice strain in integral breadth measurement. Acta Crystallogr. 1966, 20, 312−313. (49) Pang, X.; Gao, K.; Luo, F.; Emirov, Y.; Levin, A. A.; Volinsky, A. A. Investigation of Microstructure and Mechanical Properties of Multilayer Cr/Cr2O3 coatings. Thin Solid Films 2009, 517, 1922−1927. (50) Shirley, D. A. High-Resolution X-ray Photoemission Spectrum of the Valence Bands of Gold. Phys. Rev. B 1972, 5, 4709−4714. (51) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. The SIESTA method for ab-initio order-N materials simulation. J. Phys.: Condens. Matter 2002, 14, 2745−2779. (52) Mote, V. D.; Purushotham, Y.; Dole, B. N. Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles. J. Theor. Appl. Phys. 2012, 6, 6. (53) Nanda, K. K.; Behera, S. N.; Sahu, S. N. The lattice contraction of nanometer-sized Sn and Bi particles produced by an electrohydrodynamic technique. J. Phys.: Condens. Matter 2001, 13, 2861− 2864. (54) Zhang, H.; Chen, B.; Banfield, J. F. Phys. Chem. Chem. Phys. 2009, 11, 2553−2558. (55) Ivanovskii, A. L.; Shein, I. R.; Medvedeva, N. I. Nonstoichiometric s-, p- and d-metal diborides: synthesis, properties and simulation. Russ. Chem. Rev. 2008, 77, 467−486. (56) Malm, J.-O.; O’Keefe, M. A. Deceptive ″lattice spacings″ in high resolution micrographs of metal nanoparticles. Ultramicroscopy 1997, 68, 13−23.

(57) Arzac, G. M.; Rojas, T. C.; Fernández, A. Boron Compounds as Stabilizers of a Complex Microstructure in a Co-B-based Catalyst for NaBH4 Hydrolysis. ChemCatChem 2011, 3, 1305−1313. (58) Nefedov, V. I.; Gati, D.; Dzhurinskii, B. F.; Sergushin, N. P.; Salyn, Y. V. X-ray electron study of oxides of elements. Zhur. Neorg. Khim. 1975, 20, 2307−2314. (59) Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Core-electron binding energies for compunds of boron, carbon, and chromium. Inorg. Chem. 1970, 9, 612−615. (60) Mavel, G.; Escard, J.; Costa, P.; Castaing, J. ESCA surface study of metal borides. Surf. Sci. 1973, 35, 109−116. (61) Stuart, J.; Hohenadel, A.; Li, X.; Xiao, H.; Parkey, J.; Rhodes, C. P.; Licht, S. The Net Discharge Mechanism of the VB2/Air Battery. J. Electrochem. Soc. 2015, 162, A192−A197. (62) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R., Jr. NIST Standard Reference Database 20, version 4.1 (web version); http:/srdara.nist.gov/xps/, 2012. (63) Clauws, P.; Broeckx, J. Vennik, Lattice Vibrations of V2O5 Calculation of Normal Vibrations in a Urey-Bradley Force Field. Phys. Status Solidi B 1985, 131, 459−473. (64) Medvedev, E. F.; Komarevskaya, A. S. IR Spectroscopic Study of the Phase Composition of Boric Acid as a Component of Glass Batch. Glass Ceram. 2007, 64, 42−46. (65) Peak, D.; Luther, G. W.; Sparks, D. L. ATR-FTIR spectroscopic studies of boric acid adsorption on hydrous ferric oxide. Geochim. Cosmochim. Acta 2003, 67, 2551−2560. (66) Shein, I. R.; Ivanovskii, A. L. Elastic properties of mono-and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations. J. Phys.: Condens. Matter 2008, 20, 415218. (67) Zhou, C. T.; Xing, J. D.; Xiao, B.; Feng, J.; Xie, X. J.; Chen, Y. H. First principles study on the structural properties and electronic structure of X2B (X = Cr, Mn, Fe, Co Ni, Mo and W) compounds. Comput. Mater. Sci. 2009, 44, 1056−1064. (68) Li, J.; Zhang, H.; Cheng, X.; Li, D. First-principles study of the structural and electronic properties of VB2 under high pressure. Phys. B 2010, 405, 2768−2771. (69) Pereira, A. S.; Perottoni, C. A.; da Jornada, J. A. H.; Leger, J. M.; Haines, J. Compressibility of AlB2-type transition metal diborides. J. Phys.: Condens. Matter 2002, 14, 10615. (70) Balcı, S.; Sezgi, N. A.; Eren, E. Boron Oxide Production Kinetics Using Boric Acid as Raw Material. Ind. Eng. Chem. Res. 2012, 51, 11091−11096. (71) Schmidt, H. X-ray evidence for CrCO3, VBO3 and TiBO2 with calcite structure. Acta Crystallogr. 1964, 17, 1080−1081. (72) Klemm, W.; Grimm, L. Knowledge of the Lower Vanadium Oxides. Z. Anorg. Allg. Chem. 1942, 250, 42−55.

J


Effect of Surface Properties on the Microstructure, Thermal, and

Recommend Documents