Article pubs.acs.org/JPCC
Study of Surface-Active Modes and Defects in Single-Phase LiIncorporated MgO Nanoparticles G. Hassnain Jaffari,*,† Adnan Tahir,† M. Bah,‡ Awais Ali,∥ Arshad S. Bhatti,∥ and S. Ismat Shah‡,§ †
Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan Department of Material Science and Engineering and §Department of Physics, University of Delaware, Newark, Delaware 19716, United States ∥ Centre for Micro and Nano Devices, Department of Physics, COMSATS Institute of Information Technology, Park Road, Islamabad 44000, Pakistan ‡
ABSTRACT: Synthesis and detailed characterization of lithium-incorporated (0−20% of Li loading) MgO nanocrystals are presented. High surface reactivity arises due to the presence of facets and kinks at the surface of the particles. Fourier transform infrared spectroscopy (FTIR) showed that the presence of surface modes with peak shift related to the difference in the electronegativity of Li ions and the formation of extra modes for low loading of Li. For LiMgO particles, interband transitions due to the F+ centers and the surface LixO species are observed. Formation of F+ centers is consistent with a decrease in the lattice parameter as a function of Li loading. A significant increase in the magnetic moment of the LiMgO nanoparticles is discussed in terms of the presence of oxygen vacancies and Li-rich surface regions. Three new bands in the higher frequency range are observed for LiMgO nanoparticles. Overall, studies revealed that the surfaces of the particles are not the same for low and high loadings of Li.
transfer from Li to the F+ centers, which leads to formation of electron pair formation in the vacancy. In the case of MgO nanoparticles, low coordinated surface sites (MgLC2+ OLC2− pairs) can act as strong acid−base pairs. Hargreaves et al.15 demonstrated (using methane activation) that morphology of the MgO nanoparticles plays an important role for the presence of Li ions at the specific surface site. Replacing Mg2+ by Li+ creates lattice defects, like oxygen vacancies (positive hole). Moreover, the projected active-site [Li+O−] is produced by a hole neighboring to Li+ site that traps an oxygen atom.16,17 It is mostly accepted that replacing the Mg2+ by Li+ causes lattice defects (oxygen vacancies).17 Photoluminescence (PL) on MgO nanoparticles has been reported to exhibit a broad luminescence peak that is related to the presence of the surface defects.18 The emission band that peaked around 382 and 415 nm and the blue emission band at 465 nm are expected to originate from the defects in MgO (oxygen vacancies).18,19 The PL bands observed for MgO nanoparticles do not match with bulk band gap emission; the interband transitions are related to the defects. The observed bands are caused by the color centers such as F, F+, and F2+ centers.20 Photoionization measurements revealed that Li addition yields a band associated with the F+ centers and a band that is related to the surface segregation of lithium such as LixO.21 Even though LiMgO is a popular material for catalysis, systematic Raman, PL, and magnetic measurements as a
1. INTRODUCTION Magnesium oxide (MgO) is of interest due to its application as a catalyst in reactions such as activation of small alkanes,1−3 the extraction of biodiesel from vegetable oils,4 and use as a terahertz quasi-optic component.5 The surface defects in MgO nanoparticles affect the diffusion kinetics of the chemisorbed Pyridin.6 The embedded cluster model and hybrid density functional theory (DFT) revealed that the interaction of hydrocarbon and basic polar organic molecules with the MgO (001) surface can lead to design and control of molecular structure and energy levels in single-molecule electronic devices.7 Li addition promotes catalytic reactions such as methane to ethane or ethylene to other products conversions7,8 and dehydrogenation of propane.9 Li adsorption on the surface makes it an interesting system where the surface geometry and electronic structure and its role as a defect can be studied. The decrease in the work function due to the net transfer of the 2s valence electron of Li to MgO has been reported.10 Brazzelli et al. showed that the presence of Li leads to the formation of surface color centers that are paramagnetic.11 DFT calculation on Li adsorbed on the MgO surface revealed the presence of Li mainly at the terrace sites.12,13 Xu and Henkelman used DFT to show that Li monomers bind to oxygen sites on the terrace for Li clusters on MgO(100) surface.14 They presented energetics for different defect sites including oxygen vacancies, kinks, and Mg vacancies. Li forms a bond with the electron in the charged oxygen vacancy defects (F+ centers). Hence, overall binding of Li on the terrace is much stronger then Ca binding to the charged oxygen defects. Overall there is more charge density © 2015 American Chemical Society
Received: September 16, 2015 Revised: November 16, 2015 Published: November 17, 2015 28182
DOI: 10.1021/acs.jpcc.5b10131 J. Phys. Chem. C 2015, 119, 28182−28189
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The Journal of Physical Chemistry C
Figure 1. (a) X-ray diffraction patterns and (b) lattice parameters of LiMgO nanoparticles. The inset shows particle size variation as a function of Li concentration.
function of different loading fractions of Li ions have not been reported. In this work we studied LiMgO nanoparticles using various techniques. The effects of the lithium addition on the structure, lattice parameters, morphology, and average grain size have been studied. Bulk MgO is known to be Raman inactive; however, nanoparticles with surface defects exhibit drastically different results. In the present studies, the goal was to investigate band structure and vibrational modes and to identify the surface effects as a function of low and high loadings of Li ions in MgO nanoparticles. Li incorporation exhibited the presence of F+ center with an additional defect level related to the Li-rich surface regions. The FTIR modes were found to be associated with the surface carbon with a drastic change in the spectrum with Li addition. In addition to that using FTIR, bonding of less-coordinated surface ions with various C and OH species as a function of low and high loading fractions of Li has been discussed in detail.
ments were carried out at room temperature in a Quantum Design, VersaLab 3 T cryogen-free vibrating sample magnetometer (VSM). Raman spectra were recorded at room temperature using Thermo scientific DXR Raman microscope. Transmission electron microscopy (TEM) was carried out on a JEOL 2010 field-emission transmission electron microscope operated at 200 kV accelerating voltage. Diffused reflectance studies were carried out with PerkinElmer Lambda 950 spectrometer.
3. RESULTS AND DISCUSSION 3.1. Structural Characterization. X-ray diffraction (XRD) results of the pure MgO nanoparticles annealed in air at 700 °C are shown in Figure 1a. The XRD patterns were analyzed using X’Pert HighScore software. The XRD patterns correspond to a periclase cubic structure, which matches well with the ICDD pattern of MgO (ICDD no. 00-004-0829), and have been indexed accordingly. The peak corresponding to all of the reflections exhibited a shift to higher 2Θ as a function of Li loading (x ≤ 15%) fraction. The corresponding reduction in the lattice parameter as a function of Li loading (as shown in Figure 1b) seems to arise due to the presence of the oxygen vacancies. Li+ (∼76 pm) and Mg2+ (∼72 pm) have comparable ionic radii but with different valence states, which is the reason for the generation of the oxygen vacancies. As shown in Figure 1a, diffraction peaks (e.g., the 200 peaks) related to the pure MgO nanoparticles were noticeably broader then LiMgO nanoparticles. Peak broadening indicates smaller crystallite size. Average crystallite size was calculated using Scherer’s equation. To measure average sizes accurately, full width at halfmaximum (fwhm) values were calculated after striping Kα2 lines using X’Pert High Score software. The average size for samples postannealed at the same temperature (∼700 °C) for pure MgO, 5% LiMgO, 10% LiMgO, 15% LiMgO, and 20% LiMgO were calculated to be 14, 33, 37, 42, and 40 nm, respectively. The percentages here are the nominal percentages not the actual percentages. Figure 1b shows lattice parameter variation as a function of nominal Li %. Here the lattice parameter is seen to decrease as Li concentration increases, even though the Mg2+ and Li+ have comparable ionic radii. As the Li+ replaces Mg2+, formation of oxygen vacancy is expected in order to keep the charge neutrality of the crystal. The ionic radius of the oxygen is 126 pm, and formation of its vacancy explains the measured decrease in the lattice parameter. Hence, core incorporation with Li can be argued on the basis of a shift
2. EXPERIMENTAL SECTION MgO and lithium-doped MgO nanoparticles are synthesized using a sol−gel method. A high-purity magnesium acetate tetrahydrate (Sigma-Aldrich, 99.8% purity) was used as Mg and lithium acetate dihydrate was used as Li precursor. The measured amount of solute (magnesium acetate) was dissolved in 200 mL of ethanol to form a 0.1 M solution. The suspension formed is first stirred in ethanol (AnalaR, absolute, 99.9% purity) for 30 min to form a cloudy mixture. Tartaric acid (1 M, Sigma-Aldrich, 99.8% purity) was added until formation of a thick white gel. The quantity of tartaric acid added was controlled in such a way to achieve a pH of 5 to obtain the final gel; then the gel was slowly dried, and a white precursor was obtained. Postannealing at 700 °C in air was carried out for 1 h. The structural characterization of nanoparticles was carried out using PANalytical X-ray diffractometer equipped with Cu Kα radiation of wavelength λ = 0.1541 nm. The X-ray tube was operated at 45 kV and 40 mA. The photoluminescence (PL) spectroscopy was performed at room temperature by using Lab Ram III from DongWoo Optron. A He−Cd laser of wavelength 325 nm was used. The panorama scan function was used and the accumulation time for each scan was 0.5 s, while scanning from 330 to 900 nm. Incident light was controlled by using neutral density filters, and 100% of the incident power of the laser was used. The FTIR measurement was performed using Thermo Nicolet Nexus 870 FR-IR E.S.P. Magnetic measure28183
DOI: 10.1021/acs.jpcc.5b10131 J. Phys. Chem. C 2015, 119, 28182−28189
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The Journal of Physical Chemistry C in the XRD peaks. For the highest loading fraction, i.e., 20%, the lattice parameter was found to increase. The probable reasons for this increase could be the presence of additional Li ions occupying interstitial sites. The inset of Figure 1b shows average particle size variation calculated from fwhm of the most intense peak in the pattern. The average size of the crystallite increases by a factor of 2 or more compared to the pure MgO nanoparticles. 3.2. Morphological Studies. TEM was used to study the morphology, crystal size, and degree of agglomeration and to measure size distribution. Morphologically, pure MgO samples are nonspherical and the crystallite size is small compared to the Li-doped samples, as shown in Figure 2a,b. The
Figure 3. FTIR spectra of LiMgO nanoparticles.
corners or edges). The decarbonation and dehydroxylation come from the surface coordinatively unsaturated sites, which are surface oxygen anion and magnesium cation with low coordination, denoted as MgLC2+ and OLC2+. Bidentate carbonate forms on Lewis acid−Brönsted base pairs as Mn+− O2− (Mn+ is a Li+ or Mg2+ cation) and shows a symmetric (O− C−O) stretching at the band 1359−1420 cm−1 and an asymmetric (O−C−O) stretching at the band 1460−1540 cm−1. Symmetric and asymmetric O−C−O stretching bands occur at 1457 and 1647 cm−1, respectively.23,24 Some authors have also determined the following base strength order for these surface oxygen species, as low-coordination O2− anions > oxygen in Mn+−O2− pairs > OH groups.22,25 With Li addition, there is an overall shift in the position of the bands associated with the symmetric and asymmetric O−C−O stretching. The differences in the peaks with Li addition incorporation with respect to pure MgO are related to the change in the partial charge due to the difference in the valency of the Li and Mg ions.23 Here the increase in the Li concentration results in formation of oxygen ion vacancy with one electron (this will be discussed further in the Photoluminescence Spectroscopy section), which is the expected mechanism associated with the difference in the stretching energies of the CO2 bonds. The band at 855 cm−1 is related to Mg−O−Mg bending vibrations, which have been observed in all of the samples.26 Bicarbonate species formation involves surface hydroxyl groups and shows a C−OH bending mode at 1258 cm−1. This mode is absent for pure MgO and the high-loading (≥15%) fraction of Li. This mode is present in 5% Li loading, and its intensity reduces for 10% LiMgO sample as shown in Figure 3. The band observed at 1087 cm−1 is due to CO2 absorption.27 This mode is observed for the low loading of Li, and the intensity associated with the mode decreases as the Li loading increases from 5 to 10%. Similarly, an additional peak formed next to the 855 cm−1 peak was observed only for 5% and 10% Li samples. The absence of 1258 cm−1 and 1087 cm−1 and an additional peak next to the 855 cm−1 mode for the 15% and 20% loadings of Li show possible enhancement in the Li ion concentration at the surface with corresponding suppression of these modes. 3.4. Photoluminescence Spectroscopy. Figure 4a shows PL emission spectra of all of the samples. Pure MgO nanoparticles exhibited a broad peak with low intensity. It is evident from the width of the PL band that it arises due to the superposition of several peaks.18 Niu et al. showed that the PL band of MgO nanoparticles can be deconvoluted to four
Figure 2. Low- and high-resolution TEM micrographs of (a, b) pure MgO, (c, d) 10% LiMgO, and (e, f) 20% LiMgO nanoparticles, respectively.
nanocrystals are ∼10−18 nm in size and are agglomerated. The average particle size calculated from XRD was found to be ∼14 nm. The size of the particles of 10% LiMgO ranges from 35 to 100 nm as shown in Figure 2c,d. Also observed to be nonspherical are 10% LiMgO and 20% LiMgO nanocrystals. This morphology shows formation of kinks at the surfaces of the particles. The size distribution of 20% LiMgO nanoparticles ranges from 40 to 120 nm. There is a slight increase in the size of the 20% LiMgO nanoparticles compared to the 10% LiMgO nanoparticles. Again, nonspherical morphology has been observed with, e.g., five kinks for the particle shown in Figure 2e,f. However, compared to the pure MgO, all of the Li-doped samples had a significant increase in the size of the particles as seen by both TEM and XRD measurements. This seems to be related to the higher diffusion coefficient of Li in the LiMgO gel, and hence, the particle size increases. 3.3. Fourier Transform Infrared Spectroscopy. We have examined the chemical nature of adsorbed species on LiMgO nanoparticles using Fourier transform infrared spectroscopy (FTIR) and recognized three different CO2 adsorption surface species, which are unidentate carbonate, bidentate carbonate, and bicarbonate.22 Figure 3 shows the FTIR spectra of LiMgO nanoparticles. The surfaces of pure MgO and LiMgO nanoparticles are not expected to be uniform and have several species formed by CO2 adsorption (for details, see ref 22). Sample surfaces have oxygen atoms of altered chemical nature that bind with the CO2 with different binding energies and coordinations. Unidentate carbonate formation needs isolated surface O2− ions (low coordination anions, which may exist at 28184
DOI: 10.1021/acs.jpcc.5b10131 J. Phys. Chem. C 2015, 119, 28182−28189
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Figure 4. (a) Raw PL spectra of all of the samples recorded for the same amount of time. (b) Normalized PL spectra of all of the Li-loaded samples.
Gaussian peaks centered at ∼354, ∼382, ∼415, and ∼465 nm.28 These peaks are associated with oxygen vacancies due to the presence of incomplete oxidation at the surface. In addition to that, other possible defects can be interstitials, oxygen vacancies, Mg vacancies, etc. In other reports, the peaks emission bands at ∼382 and ∼415 nm and the blue emission band at ∼465 nm are expected to originate from the defects (for example, oxygen vacancies).18,19 Hence, new defect levels are expected to form within the band gap of MgO nanoparticles in an otherwise large band gap material.19 In general, formation of color centers in PL contributes to the observed spectrum as well. In general, these color centers are denoted as F center, which is oxygen ion vacancy with 2 electrons; F+ center, which is oxygen ion vacancy with one electron; and F2+ center, which is oxygen ion vacancy.20 Several authors have studied the emission and absorption spectra of the various types of color centers in pure MgO nanoparticles. The absorption bands have been reported to occur at different positions, which is due to the difference in the source energy.29 The absorption bands due to the F and F+ centers occur at 52020 and 365 nm,29 respectively, where the excitation source was the same as in the present study. Comparing this work with the information provided from earlier work shows that the max band at ∼525 nm is probably associated with F centers. The overall intensity of the PL signal for pure MgO nanoparticles is much lower compared to the Li-added compositions as shown in Figure 4. This shows that surfaces of the Li-incorporated samples contributed to the overall luminescence. For consistency and comparative purposes, the spectra were recorded for the same time interval for all of the samples. PL spectra of LiMgO nanoparticles with different loadings of Li are similar to the two bands. However, they are different compared to that for pure MgO. LiMgO nanoparticles have two peaks centered around 359 and 695 nm, as shown in Figure 4a. Comparing our results with the literature shows that the first emission band is associated with the F+ centers.20,29,30 Formation of F+ centers in Li-added samples is justifiable in the framework of formation of oxygen vacancies when Li+ ion replaces Mg2+ ions. All of the LiMgO nanoparticles have a band centered around 695 nm, as shown in Figure 4a. For gas-phase Li, it is known that the decay of the 1s2 2p1 excited state (which is 3P3/2 and 3P1/2 configuration) to the 1s2 2s1 ground state (which is 2S1/2 configuration) corresponds to the photons with wavelength of 670 nm.21 Hence, the presence of this emission peak is an indication of excess Li being present at the surfaces of the
nanoparticle. It has been shown by Myrach et al.21 that in LiMgO thin films, even for much higher temperature heat treatments, Li remains at the surface, which should be the probable case for the nanoparticles. Li is also expected to bond with oxygen at the surface, and hence, a shift in the peak position occur compared to the gas-phase case. Here it is important to point out that the band gap of the most stable Li2O compound is in the same energy range.21 Therefore, the bond with oxygen gives rise to LixO formation, which also explains the peak shift of 695 nm. Our results are consistent with observations and interpretation presented by Myrach et al.21 The band observed around 695 nm has been previously reported in LiMgO nanoparticles, which was undefined by Savoini et al.31 However, in the work reported by Myrach et al.,21 they interpreted that the diffusion of Li toward the surface and the presence of excess Li at the surface generates a PL band at 695 nm.21 The peak at 359 nm for the Li-added samples also exhibited a decrease in the peak intensity as a function of Li percentage (Figure 4a). The intensity of the first band for 5% and 10% Li concentration is comparable. However, as the loading of Li increases, ∼15% Li, the intensity of the band at 359 nm decreases (see Figure 4a). With a further increase in Li concentration to 20%, the intensity of the first band decreases further in comparison with the 15% sample. This systematic decrease in the peak intensity also seems to be related with excess Li on the surface. Figure 4b shows normalized PL spectra of LiMgO nanoparticles where the relative intensity for the 695 nm band for 20% showed an increase. However, the absence of a systematic increase in the peak around 695 nm may arise due to the generation of the other nonradiative and radiative processes in the energy range below 300 nm and above 900 nm. The luminescence peak shown in ref 21 occurs above 400 nm, being attributed to F2+ centers. However, in our case, we observed a PL (much narrower) peak around 359 nm. A recent article on the LiMgO system, where DFT calculations with experimental results on thin films were utilized, showed that the luminescence peak related to F2+ centers occurs around 550 nm, which is a different peak position compared to our work.32 Hence, comparing the peak positions with these previous results, we suggest that the PL studies reveal F+ centers in Liincorporated nanoparticles in the present studies. Furthermore, DFT calculations show that the interband is due to the F presence of centers,32 which have also been observed using both PL and reflectance measurements in the present work. 28185
DOI: 10.1021/acs.jpcc.5b10131 J. Phys. Chem. C 2015, 119, 28182−28189
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Figure 5. (a) Absorbance spectra of MgO and LiMgO samples. Kubelka−Munk plots of (b) MgO, (c) 5% LiMgO, (d) 10% LiMgO, (e) 15% LiMgO, and (f) 20% LiMgO nanoparticles.
3.5. Diffused Reflectance. To investigate the absorption properties of the nanoparticles, diffused reflectance spectroscopy was employed. The absorbance spectra of all of the airannealed samples are shown in Figure 5a. The absorbance of the various samples revealed the presence of defect states in the band gap of MgO. To further explore the edge of the defect levels, the Kubelka−Munk function (hv × F(R))2 plotted against energy has been plotted for all of the samples separately as shown in Figure 5b−f. The band gap of 7.8 eV is reported for the bulk MgO.33 Lowering of the band gap of the MgO nanoparticles due to the surface effects has also been reported in previous studies.33 The band edge of the pure MgO nanoparticles, as shown in Figure 5b, indicates a large band gap. Extrapolation has not been performed to avoid error due to the lower limit on the highest energy available to explore the edge. The spectra of the LiMgO samples are different from that of the pure MgO sample as shown in Figure 5c−f. There is evidence of the two interband transitions, as is obvious from the emergence of the two absorption edges below 6 eV for the Li-doped samples. Occurrence of these edges is consistent with the observation of the two bands in PL data for energies lower than 6 eV. These interband levels have also been consistently observed and discussed in the Photoluminescence Spectroscopy section. The edge around 3.5 eV (350 nm) is associated with F+ centers, while the band around 1.8 eV (700 nm) matches well with the phase due to the excess Li ions at the surface. 3.6. Raman Spectroscopy. The Raman spectra taken at room temperature of pure MgO and LiMgO nanoparticles are shown in Figure 6. Because of the absence of optical phonons in MgO bulk, it is the usual choice to be used as substrate.34 Raman inactivity in bulk MgO arises due to the crystal symmetry. Activity of the Raman and IR modes for pure MgO was analyzed using an online database, where the program calculated symmetry-adapted modes and yields classification for IR, Raman, and hyper-Raman active modes.35 The input parameter was the space group, which is Fm3̅m (no. 225) for MgO. Results revealed the absence of Raman-active mode for pure MgO. However, nanostructures are expected to be
Figure 6. Raman spectrum of LiMgO nanoparticles.
different due to the surface effect, which can give rise to surface modes, as reported for MgO nanotube using DFT studies.36 In the present work, we have synthesized and characterized nanoparticles where Raman peaks are observed for pure MgO nanoparticle as shown in Figure 6. A broad band peaked around 2480 cm−1 has been observed for pure MgO. The signature of a weak peak around 1088 cm−1 is also observed. This peak is attributed to the surface phonon modes.37,38 The broad band around 2480 cm−1 is composed of a strong G-band around the peak and a weak D-band at lower frequency. These peaks are reported to appear due to the carbon bonded with MgO at the surface.39 In contrast to the pure MgO, for the Li-doped samples the spectra is quite different from the pure case, and the D and G bands are there to some extent with significantly lower intensity. However, there are three strong bands at higher frequencies. They seem to arise due to the OH symmetric stretching of various modes and carbon bonds at the surface due to the presence of the Li ions at the surface. However, it is not specifically possible to identify the modes. Therefore, theoretical calculations are required. 3.7. Magnetic Properties. Defect-induced ferromagnetism in otherwise nonmagnetic oxides has been of interest.40 28186
DOI: 10.1021/acs.jpcc.5b10131 J. Phys. Chem. C 2015, 119, 28182−28189
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The Journal of Physical Chemistry C Andriotis et al.41 and Xing et al.42 discovered the magnetism in ZnO. They identified the origin of the magnetism being the defect induced. They reported room-temperature magnetism arising due to the presence of the defect in the case of samples synthesized by using the sol−gel technique. Kumar et al.43 reported ferromagnetism in MgO nanostructures and attributed it to the presence of Mg vacancies (as VMg). Similarly, many other researchers reported that the bulk MgO was nonmagnetic and MgO nanocrystals showed room-temperature ferromagnetism, which probably comes from the loss (VMg) of donor charge of O atoms, forming the 2p holes at the surface. On the basis of these calculations, Gao et al.44 studied defect-induced magnetism in MgO and proposed that the induced magnetic moment was due to spin polarization of 2p electrons of O atoms near the VMg. They suggested that the oxygen vacancies do not induce ferromagnetism. The ferromagnetism in MgO nanoparticles arises not only because of the VMg but from the positions or distribution of the vacancies as well. However, a few other studies suggested the presence of oxygen vacancies being the cause of the ferromagnetism in MgO.45,46 In addition to that, there are several experimental results that showed generation of magnetism due to oxygen vacancies, e.g., oxygen vacancies in ZnO,47 TiO2,48 CeO2,49 etc. As we have explained in the Photoluminescence Spectroscopy section, Li doping generates oxygen vacancies, as seen by the presence of F+ centers; therefore, the magnetic moment consistently increases as the Li concentration increases (as shown in Figure 7). The
utilized to characterize the nanoparticles in detail. All of the samples were crystallized in single-phase periclase cubic structure. Morphologically, particles were nonspherical in shape with facets and kinks at the surface. FTIR showed that three different kinds of CO2 species were found on the surface, as identified from the symmetric and asymmetric O−C−O stretching. The shift in band from pure to doped samples was found and was due to the difference in electronegativity and to the change in the partial charge of the surface oxygen due to the Li ions. A missing mode, which seems to be related with C− OH bending at 1258 cm−1, appeared in low loading of Li (≤10%). The peak associated with this mode had a large intensity for 5% LiMgO, which reduced for 10% LiMgO and finally vanished at higher loadings. This confirmed that the surfaces of the particles are not the same for low and high loadings of Li. Consistent variation with the appearance of interband transitions with Li loading was observed with photoluminescence (PL) and diffused reflectance spectroscopy. Broad and weak peaks associated with interband transitions for pure MgO at 525 nm were observed, which arise due to the presence of the F centers at the surfaces of the particles. However, significantly different PL spectra were recorded for the LiMgO band where the presence of F+ centers was observed with an additional band at higher wavelengths, which was assigned to the surface LixO species. Formation of F+ centers is consistent with a decrease in the lattice parameter as a function of Li loading. An increase in the magnetic moment of the LiMgO with an increase in Li loading was observed with an underlying mechanism associated with the presence of oxygen vacancies and surface-rich Li regions. It is known that no Raman modes exist for bulk MgO. The broad Raman band for pure MgO was associated with the D and G bands arising due to the surface carbon and was also found in LiMgO with significantly reduced intensity. However, three new bands were observed in the higher-frequency regions that arise due to OH symmetric stretching modes. The presence of these modes indicated significantly modified surface reactivity with Li incorporation. However, further theoretical studies are required to specifically identify the modes. Overall studies revealed that the surfaces of the particles are not the same for low and high loadings of Li, which is expected to lead to the exhibition of different catalytic activities in these systems.
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Figure 7. Room-temperature magnetization versus field behavior of LiMgO nanoparticles. The arrow indicates an increase in Li concentration.
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decreasing trend in M as a function of H is due to the weak magnetic signal and the presence of the diamagnetic holder during the measurement. Additionally, it has been reported that grain boundaries also contribute to the magnetism in wide-band semiconducting oxides.50 Similarly, it has been reported that light element doping, for example, K in SnO2, yielded phase segregation of K at the surface of the nanoparticles.51 The accumulation of K on the surfaces led to the observation of ferromagnetism. Therefore, in the present studies, the presence of LixO defects at the surface, which signature was observed by PL, seems also to be a possible reason for the observed magnetic moment.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS G.H.J. acknowledges support of the Pakistan Higher Education Commission for funding his visit to University of Delaware through the Project “Pakistan Program For Collaborative Research (PPCR)”.
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REFERENCES
(1) Ito, T.; Wang, J.; Lin, C. H.; Lunsford, J. H. Oxidative Dimerization of Methane over a Lithium-Promoted Magnesium Oxide Catalyst. J. Am. Chem. Soc. 1985, 107, 5062−5068. (2) Lunsford, J. H. The Catalytic Oxidative Coupling of Methane. Angew. Chem., Int. Ed. Engl. 1995, 34, 970−980. (3) Climent, M. J.; Corma, A.; Iborra, S.; Mifsud, M. MgO Nanoparticle-Based Multifunctional Catalysts in the Cascade Reaction
4. CONCLUSIONS In this work we present synthesis and detailed characterization of Li-incorporated magnesium oxide (LiMgO) nanocrystals. Multiple structural and spectroscopic techniques have been 28187
DOI: 10.1021/acs.jpcc.5b10131 J. Phys. Chem. C 2015, 119, 28182−28189
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