When Halides Come to Lithium Niobate Nanopowders Purity and

Article pubs.acs.org/IC

When Halides Come to Lithium Niobate Nanopowders Purity and Morphology Assistance Emmanuel Lamouroux,*,†,‡ Laurent Badie,†,‡,§ Patrice Miska,∥,⊥ and Yves Fort†,‡ †

Université de Lorraine, SRSMC, UMR 7565, Vandoeuvre Lès Nancy F-54506, France Université de Lorraine, Institut Jean Lamour, UMR 7198, Vandoeuvre Lès Nancy F-54506, France ‡ CNRS, SRSMC, UMR 7565, Vandoeuvre Lès Nancy F-54506, France ⊥ CNRS, Institut Jean Lamour, UMR 7198, Vandoeuvre Lès Nancy F-54506, France ∥

S Supporting Information *

ABSTRACT: The preparation of pure lithium niobate nanopowders was carried out by a matrix-mediated synthesis approach. Lithium hydroxide and niobium pentachloride were used as precursors. The influence of the chemical environment was studied by adding lithium halide (LiCl or LiBr). After thermal treatment of the precursor mixture at 550 °C for 30 min, the morphology of the products was obtained from transmission electron microscopy and dynamic light scattering, whereas the crystallinity and phase purity were characterized by X-ray diffraction and UV−visible and Raman spectroscopies. Our results point out that the chemical environment during lithium niobate formation at 550 °C influences the final morphology. Moreover, direct and indirect band-gap energies have been determined from UV−visible spectroscopy. Their values for the direct-band-gap energies range from 3.97 to 4.36 eV with a slight dependence on the Li/Nb ratio, whereas for the indirect-band-gap energies, the value appears to be independent of this ratio and is 3.64 eV. No dependence of the band-gap energies on the average crystallite and nanoparticle sizes is observed.

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INTRODUCTION Different common products and devices such as microphones, speakers, and accelerometers are based on the piezoelectric properties of materials that require both precision and fast response. Lithium niobate (LiNbO3) is a good candidate that presents remarkable physical properties, such as electrooptical, acoustooptical, and nonlinear-optical, besides piezoelectric properties.1,2 Recently, LiNbO3 has also been used as a bioimaging marker by taking advantage of its second harmonic generation properties.3,4 Up to now, different methods have been followed to synthesize LiNbO3 crystals and powders, mainly used to prepare ceramics, such as the combustion method, molten salt synthesis, a sol−gel process, a hydrothermal route, chemical vapor deposition, etc.5−11 However, for technology purposes, the challenge is to control both the composition and homogeneity of particles, which is difficult when hightemperature treatments are considered because volatilization of lithium could occur. Moreover, this control should be considered in the context of microelectronic component miniaturization, which requires the use of nanoparticles. The chemical route appears then to be very interesting in this context. The typical preparation of LiNbO3 nanopowdersbased on X-ray diffraction (XRD) analysisby a combustion process is © XXXX American Chemical Society

realized using lithium carbonate and hydrated niobium(V) oxide, which are mixed in the presence of urea and then heated at 850 °C for 1 h. This process leads to large squares of LiNbO3, with edge lengths ranging from 0.5 to 1.0 μm.8 The use of lithium nitrate and ammonium niobate oxalate hydrate as precursors induces the formation of aggregatesthe term “aggregates” means chemically bonded and the term “agglomerates” refers to physically bondedof primary LiNbO3 nanoparticles with sizes of 200−400 nm, resulting from a sintering process at 600 °C.9 Wohlrab et al. tried to limit the growth of LiNbO3 nanoparticles by using a confinement effect within a polymeric matrix.10 The annealing step needed to synthesize LiNbO3 and to remove the polymeric matrix also leads to the formation of aggregates. However, the primary particles constituting the aggregates are smaller and range from 50 to 200 nm, revealing a template effect of the matrix. Besides this approach, hydrothermal and/or sol−gel processes have been used to take advantage of the solvent and surfactant that are used as template agents.11 Thus, the hydrothermal route allows the formation of LiNbO3 from an alkali solution of Nb2O5·H2O heated to 250 °C for 10 h. The product presents good crystallinity coupled to the morphology of ill-defined Received: November 16, 2015

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DOI: 10.1021/acs.inorgchem.5b02638 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry flakes of 100−500 nm. Modification of the hydrothermal medium by using a quasi-reversible emulsionwith and without surfactantsyields the formation of LiNbO3 hollow spheres.12 The sizes of these spheres range from 0.2 to 1.0 μm with an acceptable crystallinity. Even if it is possible to prepare a pure LiNbO3 phase or to control the size of LiNbO3 structures at the microscale, it is still difficult to control both the crystallinity purity (i.e., the LiNbO3 phase without the Li3NbO4 phase or the LiNb3O8 phase) and the morphology size and shapefor individual nanoparticles. This difficulty is induced by a high-temperature and/or high-pressure treatment of the reaction medium. To circumvent this difficulty, we investigated here the influence of a lithium-based inorganic matrix. LiCl and LiBr have been chosen for their thermal properties and chemical compositions. In a typical experiment, we used NbCl5 pretreated in ethanol as the niobium precursor and LiOH as the lithium source with or without LiX (X = Cl or Br). Hence, the addition of LiCl does not induce chemical composition modification of the reaction medium, whereas the use of LiBr only introduces bromide anions. On the basis of transmission electron microscopy (TEM) observations, dynamic light scattering (DLS) measurements, XRD analyses, and UV− visible and Raman spectroscopies, we point out a clear matrix effect on the LiNbO3 phase purity (absence of the Li3NbO4 or LiNb3O8 phase) and nanoparticle morphology.

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diameter than when we refer to its volume or intensity.14 Indeed, if we consider a nanoparticle population with two modes (r = 1 and 10 nm, for example) and the same number of individual objects in each mode, the graph reporting the particle number as a function of the diameter will show an equal proportion of the two modes. On the contrary, if we use the volume, with particle volume V = 4/3πr3, the signal for the biggest nanoparticles will be increase compared with the one for the smaller nanoparticles by a factor of 103. Finally, the use of the intensity will induce an increase of the biggest particles by a factor of 106 because of Rayleigh’s approximation. The DLS signalreferring to a number as a function of the sizeis obtained by application of the algorithm given in eq 1. 6 1 + cos2(θ) ⎛ 2π ⎞4 ⎛ n2 − 1 ⎞ ⎛ d ⎞ ⎜ ⎟ ⎜ ⎟⎜ ⎟ 2 2 ⎝ λ ⎠ ⎝n + 2⎠ ⎝2⎠ 2R 2

I = IO

(1)

Moreover, the DLS measurement quality depends of the Z-average and polydispersity index (PDI) parameters. They are given in Table 1.

Table 1. Z-Average and PDI Parameters for the Different Samples of Nb Precursor/LiOH and of Nb Precursor/LiOH in the Presence of LiCl and LiBr sample

Z-average (nm)

PDI

Nb precursor/LiOH Nb precursor/LiOH in the presence of LiCl Nb precursor/LiOH in the presence of LiBr

1724.0 1512.0 789.5

0.335 0.315 0.355

EXPERIMENTAL SECTION Like the overall average size, it is worth noting that the Z-average size can only be directly used if the sample is monomodal, spherical, and monodisperse. In any other case, this size can only serve as a comparison of the sample results measured under the same conditions. Diffuse-reflectance UV−visible spectra were recordedresolution of 1 nmat room temperature with a PerkinElmer LAMBDA 1050 equipped with an integrating sphere (diameter of 60 mm). For analysis, the samples were directly placed in a powder sample holder. The Tauc relationship (eq 2) has been used to determine the optical band gaps,15,16

1. Chemicals. Niobium pentachloride (NbCl5) from Strem Chemicals was freshly sublimed before use, and lithium chloride (LiCl; 99%, Strem Chemicals), lithium bromide (LiBr; 99%+, Strem Chemicals), and ethanol (EtOH; technical grade) were used as received without further purification. 2. Synthesis of Lithium Niobate (LiNbO3). The desired amount of NbCl5 was poured in EtOH and stirred for 5 min. The solution was filtered at a size of 0.45 μm, and the solvent was removed by evaporation at 70 °C overnight to give the Nb precursor. Then, 1 equiv of LiOH with/without 10 equiv of lithium halide (LiCl or LiBr) was mixed with the Nb precursor using a mortar for 5 min. Then the mixture was placed in a quartz boat and annealed at 550 °C. After 30 min, the furnace was switched off and the temperature then decreased to room temperature. The white solid as obtained was rinsed with EtOH and centrifuged three times (10000 rpm for 10 min) to remove lithium halide. Finally, the product was dried in a furnace at 70 °C overnight. This procedure was repeated three times for each chemical environment. 3. Characterization. Powder XRD measurements of the samples were performed at room temperature using a Philips Xpert Pro powder X-ray diffractometer (Cu Kα1 radiation). The experimental diffractograms were compared to JCPDS 85-2456. Determination of the apparent crystallite size was carried out using the profile-matching mode (Le Bail) of the Fullprof programusing the function Npr = 7 and the size model 16.13 For TEM analysis and DLS measurements, suspensions of 1 mg/L LiNbO3 in EtOH were prepared. The suspension was sonotroded at 110 W for 90 s in a pulsed regime (0.1/0.1 cycling time) corresponding to 750 J/mL. Finally, this as-prepared suspension was put directly into the polystyrene cell (1 cm) for DLS measurements. For the TEM sample, 2 droplets of the suspensiondescribed abovewere drop-dried onto a 200-square-mesh copper TEM grid with a carbon film. TEM observations were performed on a Philips CM200 transmission electron microscope operating at an accelerating voltage of 200 kV. DLS was carried out with a ZetaSizer NanoZS (Malvern Instruments) to evaluate the LiNbO3 cluster size in an EtOH suspension. It is worth noting that the description of the suspensions is better when we refer to the object number as a function of the

[F(R ) hν]1/ n = A(hν − Eg )

(2)

where A is a constant (independent from ν), hν is the photon energy, Eg is the allowed energy gap, n is a parameter depending upon the quantum selection rules for a specific material, and F(R) is the Kubelka−Munk parameter, also known as the remission function. Equation 3 gives the correspondence between the remission, F(R), and the reflectance, R.

F(R ) =

(1 − R )2 2R

(3) 1

In eq 2, n equals /2 for an allowed direct electronic transition during absorption and 2 for an allowed indirect electronic transition. Chemical analyses to determine the ratio Li/Nb were carried out by the Service d’Analyze des Roches et des Minéraux (SARM) of the Centre de Recherches Pétrochimiques et Géochimiques in Vandoeuvre Lès Nancy. Atomic absorption spectroscopy (Varian 220FS flame) technique was used for the lithium element and the inductively coupled plasma mass spectroscopy (Thermo Elemental X7) technique for the niobium element; the samples were prepared by alkali fusion. Raman spectroscopy was realized using a LabramHR−Jobin Yvon spectrometer. We used a 633 nm excitation wavelength, a 1200 lines/ mm grating, and a cooled CCD camera for detection. The spectrometer was calibrated using a silicon wafer following the procedure described elsewhere.17 For analysis, the sample was spread out on a glass substrate. The homogeneity of the samples was controlled by recording Raman spectra at different locations at the surface. B


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Inorganic Chemistry

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RESULTS AND DISCUSSION The preparation of lithium niobate implies the annealing of a niobium source in the presence of another source of lithium. This treatment at temperatures higher than 500 °C leads to the formation of lithium niobate primary particle aggregates. In order to avoid this aggregation phenomenon, we assumed that the addition of an inorganic saltlithium chloride or lithium bromidecould be beneficial by playing the role of a matrix effect. Thus, we conducted lithium niobate formation from three different mixtures of reagents: (i) Nb precursor/LiOH, (ii) Nb precursor/LiOH in the presence of LiCl, and (iii) Nb precursor/LiOH in the presence of LiBr, where the Nb precursor corresponds to a filtered solution of NbCl5 poured into EtOH. The detailed morphology of our products was characterized by TEM (Figure 1). Annealing of a Nb precursor/LiOH

distributions also follow Gaussian laws: (i) 31.2 nm (σ = 20.2) and 118.5 nm (σ = 36.1) and (ii) 85.3 nm (σ = 77.8) and 282.3 nm (σ = 13.1), respectively. The presence of LiCl allows the growth of smaller nanoparticles than those with LiBr. The product morphology shows a clear dependence on the chemical environment preparation. The diameters of sample suspensions in EtOH obtained by DLS measurements are reported in Figure 2. This corresponds

Figure 2. DLS measurements for the materials obtained from Nb precursor/LiOH (circles), Nb precursor/LiOH in the presence of LiCl (squares), and Nb precursor/LiOH in the presence of LiBr (tilted squares) at 550 °C for 30 minsuspension of 1 mg/mL of the sample in EtOH. Number versus hydrodynamic diameter in nanometers (Figure S2 in the Supporting Information portrays the intensity versus hydrodynamic diameter).

to the hydrodynamic diameter of the system in solution (individual nanoparticles and/or clusters of primary particles). Thus, a maximum at 539.3 nm is measured for Nb precursor/ LiOH, whereas two maxima were found for both the sample obtained in the presence of LiCl and the one obtained in the presence of LiBr at 221.7 and 1281.6 nm and at 351.8 and 1506.4 nm, respectively. These DLS measurements could then be separated into two classes: one with only one mode and (ii) another exhibiting two modes. Even if these results confirm statistical analysis in terms of the population number per sample, they also point out that the nanoparticles observed by TEM are not individual objects. The formation of clusters of primary particles should then be induced by a sintering process during the annealing step. On the one hand, DLS measurements also show that the presence of an inorganic matrixLiCl or LiBrdecreases the lithium niobate cluster size. On the other hand, it also induces the formation of bigger clusters. This observation can be explained in terms of the Nb precursor concentration within the inorganic matrix: low/high concentrations lead to the formation of small/big clusters, respectively, as obtained by an aggregation process. Structural characterization of the different samples by XRD reveals that all of the samples present a pure lithium niobate phase (Figure 3). All of the diffraction peaks have been indexed with the rhombic phase of LiNbO3.18 The lattice parameters of this phase are a = b = 5.1485 Å and c = 13.8581 Å. Figure 3 highlights the purity of the LiNbO3 phase and the repeatability of the procedure of LiNbO3 preparation. The average crystallite sizes of different samples were determined (Table S1 in the Supporting Information) without lithium halide saltNb precursor/LiOHto be of 42.5 nm. In the presence of LiCl, the value yields 44.4 nm and reaches 45.7 nm in the presence of LiBr. By taking into account the standard

Figure 1. Top: TEM micrographs of the materials obtained from (a) Nb precursor/LiOH, (b) Nb precursor/LiOH in the presence of LiCl, and (c) Nb precursor/LiOH in the presence of LiBr at 550 °C for 30 min (scale bar 100 nm). Inset: Micrograph of the sample at lower magnification. Bottom: Size distributions of primary particles for the sample prepared from (d) Nb precursor/LiOH, (e) Nb precursor/ LiOH in the presence of LiCl, and (f) Nb precursor/LiOH in the presence of LiBr at 550 °C for 30 min.

mixture at 550 °C for 30 min leads to the formation of large objects of up to 0.5−0.6 μm. These large objects have illdefined morphology and seem to give birth to a sintering process during annealing. When LiCl is used as the inorganic matrix, the material consists of primary particles with diameters of 14−195 nm arranged in large clusters (≈1−2 μm). Moreover, these particles are more or less cubic with clearly pronounced edges. Nevertheless, if LiBr serves as the inorganic matrix, the nanoparticles have a rounded shape with a size ranging from 15 to 370 nm. In this latter case, TEM observation did not show any assembly of the nanoparticles into a large cluster. Statistical analysis of the size distribution of the particles for different samples highlights that the addition of the inorganic matrix yields the presence of two nanoparticle populations. Without the inorganic matrix Nb precursor/LiOH, the nanoparticle population follows a Gaussian distribution (55.9 nm; σ = 28.6). It is worth noting that a Gaussian distribution corresponds to a growth process by successive adatom to nuclei and/or nanoparticles, whereas a growth process by particles coalescence leads to a size distribution adopting a log-normal law. For the samples obtained using LiCl or LiBr, the size C


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Inorganic Chemistry

Figure 4. Dependence of [F(R) hν]2 (top) and [F(R) hν]1/2 (bottom) on the incident photon energy (hν) for the materials obtained from the Nb precursor (dashed line) and samples obtained from Nb precursor/LiOH (circles), Nb precursor/LiOH in the presence of LiCl (squares), and Nb precursor/LiOH in the presence of LiBr (tilted squares) at 550 °C for 30 min. (For more clarity, only one sample obtained from each chemical environment is shown.)

Figure 3. (Top) Typical XRD diffractograms of materials obtained from Nb precursor/LiOH (circles), Nb precursor/LiOH in the presence of LiCl (squares), and Nb precursor/LiOH in the presence of LiBr (tilted squares). (Bottom) XRD diffractograms obtained after repeating the process three times for samples prepared from Nb precursor/LiOH in the presence of LiBr (hexagons, tilted squares, and triangles). All of the samples were prepared at 550 °C for 30 min. The solid lines correspond to the JCPDS 85-2456 database standard.

LiBr belong to this class. For the second one, two edges could be observed for the same part of the curvesone around 1.5− 1.7 eV and the other one around 2.5 eV. These signals can result from thermally induced small polaron NbLi3+ and bipolaron NbLi3+−NbLi4+ in LiNbO3.19 Additionally, because this particularity concerns the samples Nb precursor and Nb precursor/LiOH in the presence of LiCl, these two edges can also be attributed to some unreacted Nb precursor present in Nb precursor/LiOH in the presence of LiCl or the same persistent impurity. The baseline for these two samples corresponds to the straight line between these two edges. For the materials obtained in the presence of LiBr, two band gaps can be extracted from the curve [F(R) hν]1/2 versus hν. The one at 3.64 eV corresponds to an indirect allowed electron transition for the lithium niobium material. The other one yields 3.15 eV, which could correspond to a Nb precursor residue and/or Nb2O5not revealed by XRD with energies of 3.11 and 3.07 eV, respectively.20 Moreover, EDS analysis of the different samples did not reveal the presence of halide (Figure S1 in the Supporting Information). This lower value could also be attributed to the formation of NbLi4+ in Lienriched LiNbO3 or a Q-polaron defect resulting from the reduction process allowed by an impurity presence during LiNbO3 synthesis.21,22 Because the band-edge shift of UV− visible spectra is dependent on the crystal stoichiometry, the Li/Nb ratio in the LiNbO3 samples can be estimated using UV−visible spectra in the case of crystal samples.23 The different useful parameters are the absorption coefficient, thickness of the sample, transmission, and refractive index. Additionally, where nanopowders are concerned, chemical analyses need to be performed to determine the Li/Nb molar ratio (Table S1 in the Supporting Information).

deviation obtained for the crystallite sizes, this parameter does not appear to be dependent on the chemical environment. Moreover, on the basis of XRD analysis, independent of the chemical environment, the materials present a pure crystalline phase of LiNbO3. As previously mentioned, DLS measurements point out that the samples in solution consist of bigger objects (clusters of primary particles) than those observed by the TEM technique. Additionally, a comparison of the average crystallite sizes and TEM nanoparticle sizes allows one to conclude that different samples consist of two types of populations: one that is monocrystallinethe nanoparticle size is close to the average crystallite sizeand a second one that is polycrystalline and has a bigger average particle size. For our samples, the direct-band-gap energies were evaluated from the plot of [F(R) hν]2 versus hν and the indirect-band-gap energies from the plot [F(R) hν]1/2 versus hν (Figure 4). The band-gap values were obtained at the intersection of the straight lines in the linear region and the baseline. For direct allowed electron transitions, the following average values of Edg are obtained: 4.22 eV (standard deviation: σ = 0.04), 4.09 eV (σ = 0.01) and 4.15 eV (σ = 0.19), respectively, for the samples obtained from Nb precursor/LiOH alone and Nb precursor/LiOH in the presence of LiCl and LiBr. When indirect allowed electron transitions are concerned, the values appear to be more homogeneous and are 3.64 eV (σ = 0.02) for the three kinds of LiNbO3 samples. Analysis of the part before the UV−visible edge allows, however, one to classify the samples into two classes. For the first one, the low hν value partbelow ≈3 eVis relatively straight: the samples Nb precursor/LiOH and Nb precursor/LiOH in the presence of D


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Inorganic Chemistry Figure 5 shows the plot of the direct band gap (left) and indirect band gap (right) versus the Li/Nb ratio of each sample.

Figure 5. Dependence of the direct band gap (left) and indirect band gap (right) on the Li/Nb molar ratio for samples obtained from Nb precursor/LiOH (circles), Nb precursor/LiOH in the presence of LiCl (squares), and Nb precursor/LiOH in the presence of LiBr (tilted squares) at 550 °C for 30 min. Figure 6. Raman spectroscopy. (Top) (a) A1TO, (b) A1LO, (c) ETO, and (d) ELO modes from ref 31. Bottom: Raman spectra recorded for a Nb precursor (dashed line) and samples obtained from Nb precursor/LiOH (circles), Nb Precursor/LiOH in the presence of LiCl (squares), and Nb Precursor/LiOH in the presence of LiBr (tilted squares) at 550 °C for 30 min. The asterisk refers to LiNb3O8.

It is worth noting that the lithium-rich phase boundary limit of crystalline LiNbO3 corresponds to a Li/Nb ratio yielding ≈1.02. So, it is surprising to obtain three samples that exhibit Li/Nb ratio values higher than 1.1, which correspond to a lithium enrichment. This enrichment is usually accompanied by a decrease of the oxygen content, resulting in the formation of the phase Li4NbO3. However, this crystalline phase is not observed on XRD diffractogramms. For direct allowed electron transition, in the case of Nb precursor/LiOH (dashed circle, Figure 5, left) and in the presence of LiCl samples, the bandgap value does not appear to be Li/Nb-ratio-dependent. For Nb precursor/LiOH in the presence of LiBr, the direct-bandgap values linearly depend on the Li/Nb ratio (dashed line, Figure 5, left). Surprisingly, indirect-band-gap values appear to be independent of both the Li/Nb molar ratio and the chemical environment used for the preparation of the LiNbO3 samples (Figure 5, right). Moreover, it is worth underlining that there is no dependence between the average crystallite size and both direct- and indirect-band-gap energies (Table S1 in the Supporting Informations). Theoretical consideration leads to a band gapslightly indirectof 4.9 eV, which is higher than the values obtained for ferro- and paraelectric lithium niobate by Thierfelder et al. by using the GW (FLAPW) method and with gaps of 4.7 and 4.2 eV, respectively.24,25 On the basis of these theoretical results, the values obtained for the indirect band gap appear aberrant. However, DFT calculations indicated that the indirect band gaps for para- and ferroelectric lithium niobate are predicted to be at around 2.52−2.57 and 3.48−3.61 eV, respectively.25,26 Moreover, a comparison of our results with DFT theoretical ones indicates that our samples have a bandgap value closer to the ferroelectric phase than to the paraelectric one. Experimental determinations of the lithium niobate band gap based on UV−visible spectroscopy give values ranging from 3.28 to 3.94 eV for the indirect band gap27−30 and from 3.80 to 4.12 eV for the direct band gap, respectively.20,28,30 Hence, our values3.64 eV for the indirect band gap and from 3.97 to 4.36 eV for the direct band gapare in agreement with those previously reported. Raman spectroscopy is a sensitive tool for characterizing lattice deformations and checking for the presence of defects. Lithium niobate is a rhombohedral R3c ferroelectric phase at room temperature, which presents 9E + 4A1 active Raman modes. The different observable modes for our samples are labeled in Figure 6. Raman spectra show then that all of the

samples have the same main signature attributed to lithium niobate: A1(TO), A1(LO), E(TO), and E(LO) modes.31−36 Considering the different areas of the spectra, changes can be observed between the three products. Peaks below 150 cm−1 and at around 700 cm−1 are attributed to the LiNb3O8 phase.35 It is worth noting that the absence of this phase in the XRD diffractogram can be attributed to its low proportion within the sample and/or to a too small average crystallite size. Thus, in the absence of an inorganic matrixNb precursor/LiOHthe obtained material is mainly constituted by LiNbO3 with the presence of LiNb3O8, which shows a deficiency in the lithium concentration. The presence of these two phases should be interpreted on account of an inorganic salt state at 550 °C. The melting point of lithium hydroxide, LiOH, is around 450−460 °C.37,38 Hence, in the absence of an inorganic matrix, the LiOH vapor pressure leads to a leak of the lithium precursor, whereas in the presence of inorganic salts, lithium contained in LiOH could interact with halide centers and, in this way, stays close to the niobium precursor ready for the diffusion process. It is worth noting that blank experimentsNb precursor/LiCl and Nb precursor/LiBr at 550 °C for 30 mindo not allow the formation of LixNbyOz oxides (with x, y and z ≥ 1). The material prepared in the presence of LiCl or LiBr exhibits only a LiNbO3 characteristic signal. However, the use of LiBr during the annealing step shows a more detailed signal of LiNbO3 than in the presence of LiCl. So, bromide appears to be more beneficial than chloride in reaching good LiNbO3 phase purity.

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CONCLUSIONS In this study, we evidenced that the addition of inorganic saltsLiCl or LiBrto the reaction mediaNb precursors/ LiOHstrongly influences the composition and morpholgy of the product. Thus, TEM micrographs clearly indicate that inorganic salt addition induces the presence of two nanoparticle populations. For samples obtained using LiCl, the two population sizes yield 31.2 and 118.5 nm and 85.3 and 282.3 nm for LiBr use, respectively. This trend is confirmed by DLS E


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Inorganic Chemistry

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measurements. Additionally, with LiCl the particles are more or less cubic with clearly pronounced edges, whereas with LiBr, rounded shape nanoparticles are obtained. The direct- and indirect-band-gap energies of the samples have been determined and range from 3.97 to 4.36 eV for the direct band gap and yield 3.64 eV for the indirect one, respectively. Only the direct-band-gap energies are slightly dependent on the Li/Nb ratio. For our samples, both the direct- and indirectband-gap energies are not dependent on the crystallite/ nanoparticle sizes. Moreover, Raman spectroscopy highlights that the three samples prepared here do not exhibit the same purity. The addition of LiBr to the reaction medium appears to be necessary to obtain the purest lithium niobate.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02638. Average crystallite size, Li/Nb molar ratio, and band-gap energies for the different samples (Table S1), EDS analysis of Nb precursor/LiOH in the presence of LiBr (Figure S1), and DLS measurements for various materialsIntensity versus hydrodynamic diameter (Figure S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

L.B.: Laboratoire de Chimie des Polymères organiques, UMR5629, Université de Bordeaux/CNRS/IPB, Allée Geoffroy Saint Hilaire, Bâtiment B8, CS50023, 33615 Pessac Cedex, France.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the French National Agency for Research (Grant ANR-08-NANO-041) and labeled by the French competitiveness clusters PLASTIPOLIS and MATERALIA. The authors thank Prof. M. François, G. Medjahdi from the Institut Jean Lamour, and J. Marin from the SARM of the Centre de Recherches Pétrochimiques et Géochimiques for fruitful discussions.

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REFERENCES

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