Synthesis, Structure, and Luminescence Properties of K2Ba7Si16O40

Feb 17, 2014 - Furthermore, the phosphor exhibits high quantum efficiency and good thermal stability, indicating that the K2Ba7Si16O40:Eu2+ phosphor c...
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Synthesis, Structure, and Luminescence Properties of K2Ba7Si16O40:Eu2+ for White Light Emitting Diodes Wenzhen Lv,†,‡ Yongchao Jia,†,‡ Qi Zhao,† Wei Lü,† Mengmeng Jiao,†,‡ Baiqi Shao,†,‡ and Hongpeng You*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ Graduate University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, a novel divalent europium-doped green emitting blue convertible K2Ba7Si16O40:Eu2+ phosphor has been synthesized successfully via high-temperature solid-state reaction. The structure of K2Ba7Si16O40:Eu2+ has been refined on the basis of X-ray powder diffraction by Rietveld refinement method, which consists of two infinite dimensional layers of cornersharing [SiO4] tetrahedral parallel to (201) linked together through eight-coordinate K and seven- and nine-coordinate Ba atoms. Under excitation of blue light, the K2Ba7Si16O40:Eu2+ phosphor emits a strong green emission band peaking at 500 nm. The concentration quenching mechanism of the Eu2+ ion was discussed and the critical distance was calculated. Furthermore, the phosphor exhibits high quantum efficiency and good thermal stability, indicating that the K2Ba7Si16O40:Eu2+ phosphor can serve as an effective green emitting candidate for blue light down-conversion white light emitting diodes. type structures as LaSr2AlO5:Ce3+ or Sr3AlO4F:Ce3+ successively. The LaSr2AlO5:Ce3+ can be comprehended as the substitution of the Sr−Si bond by the La−Al bond and the Sr3AlO4F:Ce3+ is the Si−O bond replaced by the Al−F bond.7,8 And Seshadri and co-workers even synthesized solid solutions between the nearly isostructural compounds Sr3AlO4F and Sr3SiO5 and reported its luminescence properties.9 Although all the derivatives of garnet or Cs3CoCl5 family have shown excellent luminescence properties, they are only further extending the traditional phosphor system. As a result, the limited investigation on the known system may restrict the development of the current phosphors. Thankfully, some researchers changed this situation and reported some oxynitride or nitride phosphors with highly efficient luminescence.10−14 Because the nitrogen exhibits more varied cross-linking patterns in oxynitride or nitride, these compounds have a novel atoms package method compared with the traditional oxide system. However, the critical reaction conditions and the increasing

1. INTRODUCTION Recently, white light emitting diodes (WLEDs) have been drawing more and more attention as solid-state light sources all over the world. The main reason behind the phenomenon is their significantly reducing global power requirements and their use as fossil fuels.1,2 The most common way to realize WLEDs is the combination of blue chip with yellow or green and red phosphors. Therefore, many phosphors have been investigated for WLEDs. Among these phosphors, the most famous yellow phosphor is Y3Al5O12:Ce3+, which is an artificial compound developed from {C}3[A]2(D)3O12 garnet system.3 It shows excellent photoluminescence properties under blue light excitation. Since luminescent properties may be determined by structure, many researchers have spent a lot of effort with garnet and developed some phosphors with garnet structure s u c h a s L u 2 C aM g 2 ( S i , G e ) 3 O 1 2 : C e 3 + ( y e l l o w ) o r Ca3Sc2(SiO4)3:Ce3+ (green).4,5 Another notable yellow phosphor is Sr3SiO5:Ce3+, which also shows great potential application in blue WLEDs.6 It belongs to the Cs3CoCl5 family with a tetragonal structure (space group P4/ncc). Recently, people reported some derivatives closely related to the Sr3SiO5© 2014 American Chemical Society

Received: January 20, 2014 Published: February 17, 2014 4649

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production costs are negative factors in their wide application from the viewpoint of industrial applications. Just recently, a new Eu2+-doped (Ca1−x−y,Srx,Euy)7(SiO3)6Cl2 phosphor with a base-cantered monoclinic lattice shows a broad emission band peaking at 580 nm.15 Meanwhile, Pan and co-workers also reported a new yellow Ba0.93Eu0.07Al2O4 phosphor under the excitation at 440−470 nm.16 These results inspire us to explore diversity compounds with new crystal structures and develop novel phosphors with higher efficiency, better color rendition, and chemically and physically stable characteristics. The compound silicate K2Ba7Si16O40 was first reported by Dent Glasser and co-workers; however, no investigation regarding luminescence properties of K2Ba7Si16O40 has been reported. 17 Herein we report a new green silicate K2Ba7Si16O40:Eu2+ phosphor after many failed attempts in silicate compounds. We studied the crystal structure, luminescence properties, concentration quenching, quantum efficiency, and the thermal stability of this novel phosphor in detail.

Figure 1. Observed (cross), calculated (solid line), and difference (bottom) results of XRD refinement of K2Ba6.93Si16O40:0.07Eu2+. Bragg reflections are indicated with tick marks. (Symmetry, tetragonal; space group, C2/m; a/Å = 32.046(7); b/Å = 7.705(6); c/Å = 8.224(6); Z = 2; RP = 10.16%; RWP = 13.56%).

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. K2Ba7(1−x)Si16O40:7xEu2+ compositions were synthesized by high-temperature solid-state reaction. The starting materials were highly pure KNO3 analytical reagent (A.R.), Eu2O3 (99.99%), BaCO3 (A.R.), and SiO2 (A.R.). Stoichiometric amounts of starting materials were ground well, subsequently placed in a crucible, and fired under reducing conditions (20% H2 + 80% N2) at 1100−1200 °C for 4 h. Then the crucibles cooled to room temperature and the as-synthesized samples were ground in an agate mortar. 2.2. Characterization. Powder X-ray diffraction (XRD) measurements were performed on a D8 Focus diffractometer (Bruker) operating at 40 kV and 40 mA. The GSAS program was used for the structural refinements. The diffuse-reflectance spectra were obtained by a Shimadzu UV-3600 UV−vis−NIR spectrophotometer with the reflection of black felt (reflection 3%) and white BaSO4 (reflection 100%) in the wavelength region of 200−800 nm. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the obtained powders were recorded with an Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Absolute photoluminescence quantum yields were measured by absolute PL quantum yield measurement system (C9920-02, Hamamatsu Photonics K. K., Japan). The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width, 4 ns; gate, 50 ns) as the excitation source (Continuum Sunlite OPO). The temperature-dependent properties of the phosphors were measured with a HoribaJobin−Yvon Fluorolog-3 FL3-211 spectrometer equipped with a 450 W xenon lamp as the excitation source. All the measurements were performed at room temperature.

fitting quality. And the refined lattice parameters are a/Å = 32.046(7); b/Å = 7.705(6); c/Å = 8.224(6), V/Å3 = 1994.825, which is in agreement with the result from ref 17. Figure 2a shows the three-dimensional structure of K2Ba7Si16O40 unit cell viewed from the b-direction. The K ions and Ba ions, arranged at the interstitial positions between the infinite layers, can be seen clearly in the figure. For a clear representation of the layer structure, the two-dimensional layer has been picked out as Figure 2b shows. One can see clearly the layer is composed of corner-sharing [SiO4] tetrahedral which is parallel to (201) and extends infinitely along the b-direction. Specially, there are five distinct crystallographic sites of Ba2+ ions with coordination numbers varying from 7 to 9 as Figure 2c depicts. The Ba(1) and Ba(3) have a higher coordination number of 9 and a similar geometrical arrangement. The Ba(2) has a coordination number of 7 with O atoms. The coordination of Ba(4) and Ba(5) consist of eight O atoms. Finally, the Ba(5) has a more regular coordination with eight O atoms.17 On account of the ionic radii and valence, the Eu2+ ions are expected to randomly occupy the Ba2+ ions. The five irregularly coordinated Ba2+ ions typically provide a diverse crystal environment for the Eu2+ ions. 3.2. Diffuse Reflectance Spectra. Figure 3 shows the diffuse reflection spectra of the undoped and Eu2+-doped K2Ba7Si16O40 samples. The undoped sample shows pure white body color, a strong drop in reflection in the UV range below 260 nm, which is assigned to the electronic transition from the valence band composed of O2p to conduction band formed by metal orbitals dominantly. As a result, the optical band gap for host lattice absorption can be extrapolated from the Kubelka− Munk function,18 F(R ) = (1 − R )2 /2R = K /S

3. RESULTS AND DISCUSSION 3.1. Crystal Structure of K2Ba7Si16O40:Eu2+. The Rietveld refinement of K2Ba6.93Si16O40:0.07Eu2+ has been preceded by using K2Ba7Si16O40 as a starting model, which crystallizes in a monoclinic unit cell with space group C2/m. The observed and calculated X-ray diffraction patterns as well as the difference profile of the Rietveld refinement for K2Ba6.93Si16O40:0.07Eu2+ samples have been illustrated in Figure 1. The obtained profile factors are RP = 10.16% and RWP = 13.56%, revealing a good

where K represents the absorption coefficient, S represents the scattering coefficient, and R represents the reflectivity. As a result, the optical band gap value can be obtained by calculating the Kubelka−Munk function to K/S = 0. Therefore, for the undoped samples, there is only one absorption band with a peak center at about 250 nm and its optical band gap is estimated to be about 3.5 eV (354 nm). For the Eu2+ ion-doped samples, several strong absorption bands located at 375, 400, 4650

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absorption range in the Eu2+ ion-doped samples is almost consistent with the excitation spectra which will be discussed in the next section. 3.3. Luminescence Investigations of K2Ba7Si16O40:Eu2+. Figure 4 shows the PLE and PL spectra

Figure 4. PLE and PL spectra of K2Ba6.72Si16O40:0.28Eu2+ phosphor.

of the K2Ba6.72Si16O40:0.28Eu2+ phosphor. The PLE spectrum is composed of four bands I (∼290 nm), II (∼319 nm), III (∼373 nm), and IV (∼440 nm). The PL spectrum of K2Ba6.72Si16O40:0.28Eu2+ consists of one broad nonsymmetrical band with a peak at about 500 nm and a tail at the longwavelength side, which is caused by the electric dipole allowed 4f65d1−4f7 transition of the Eu2+ ions. The full-width at halfmaximum of the band is about 3082 cm−1. The nonsymmetry of the PL emission band may be attributed to the energy transfer between the different Eu2+ ions sites in the host. Figure S1 in the Supporting Information shows the normalized PL spectra of K2Ba7Si16O40:Eu2+ samples with various Eu2+ contents. A small red shift and a little broadening of the emission band can be observed upon raising the Eu2+ concentration. The red shift is caused by two possible reasons as the reabsorption process or energy transfer between different sites of the Eu2+ ions. The former process is an interaction result of the high-energy part in the emission spectrum resonant with the low-energy part in the excitation spectrum. Thus, the high-energy part can be reabsorbed, which makes the emission spectrum shift to longer wavelength and show a little broadening sometimes. As to the energy transfer between different Eu2+ ions, energy is transferred from high to low energy. From the former structure analysis, it is known that there are five crystallographic Ba sites that can be occupied by Eu2+ ion in the host. To distinguish the different luminescence sites of the Eu2+ ions, the emission curve has been well-fitted with a sum of five Gaussian functions successfully as depicted in Figure S2.19 The five Gaussian bands with different intensity roughly have maxima at about (band I) 471 nm, (band II) 487 nm, (band III) 505 nm, (band IV) 532 nm, and (band V) 577 nm, respectively. Possibly, the intensity diversity is caused by the relative occupation numbers of the different sites. The energetic position of the 5d-band edges (E) for the Eu2+ ion is sensitive to electron−electron repulsion between the center ion and its surrounding anions. Then the relationship between the emission maxima positions and the Eu2+ ions sites in the host can be deduced from the VanUitert equation20

Figure 2. Crystal structure of 1 × 1 × 1 unit cells of K2Ba7Si16O40 (a). The infinite two-dimensional layers of corner-sharing [SiO4] tetrahedral seen from (201) (b). Coordination geometry of anions around the Ba2+ ions (c).

Figure 3. Reflectance spectra of K2Ba7(1−x)Si16O40:7xEu2+ (x = 0, 0.01, 0.02, 0.04, 0.06).

and 440 nm together with the main host lattice absorption band can be obviously seen from the reflection spectra so that the phosphors show green body color. These adding bands are attributed to the absorption of the Eu2+ ions. The broad 4651

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⎡ ⎛ V ⎞1/ V ⎤ E(cm−1) = Q *⎢1 − ⎜ ⎟ ⎥ × 10−(nEar)/80 ⎝ 4 ⎠ ⎥⎦ ⎢⎣

where E is the position for the Eu2+ ion emission peak, Q* is the position in energy for the lower 5d-band edge for the free Eu2+ ion (Q* = 3400 cm−1), V is the valence of the Eu2+ ion (V = 2), n is the number of anions in the immediate shell about the Eu2+ ion, Ea is the electron affinity of the anions (eV), and r is the radius of the host cation replaced by the Eu2+ ion (Å). Ea is a constant in the same host and V = 2. On the basis of the above analysis, we know that the value of E is directly proportional to the product of n and r. The Ba(1) and Ba(3) atoms are nine-coordinated with the Ba−O band distance of about 2.870 and 2.900 Å. The Ba(2) atoms are sevencoordinated by O atoms at an average Ba−O distance of 2.799 Å. The coordination of Ba(4) and Ba(5) consist of eight O atoms and their average bond distances are 2.913 and 2.845 Å, respectively. Considering these data, it is reasonable to speculate that the band I (∼471 nm) corresponds to Ba(2). The band II (∼487 nm) should be from the Ba(5) center. The band III (∼505 nm) belongs to the excitation bands of the Ba(4) center. And the band IV (∼532 nm) and band V (∼577 nm) are attributed to the Eu2+ ion in Ba(1) and Ba(3), respectively. The Stokes shift can be estimated by taking twice the energy difference between the zero phonon line energy and the energy of the emission maximum.16,21 The spectral position of the zero-phonon line can be estimated from the intersection of absorption and emission spectra. Using the intersection point (460 nm) between the excitation and emission curves, the lowest excited state of the 7FJ multiplet of the 4f6 configuration (zero-phonon line) is determined to be 2.7 eV. Therefore, the Stokes shift of the Eu2+ emission is evaluated be 0.44 eV. The Stokes shift of the Eu2+ ions in K2Ba7Si16O40 is very similar to the Stokes shift of the Eu2+ ion in Ca-a-SiAlON (0.44 eV) and the SrSi2O2N2 (0.42 eV).21,22The smaller Stokes shift of oxynitride or nitride is always caused by the high rigidity of the networks in the host lattice. Therefore, in our case, the layer composed of corner-sharing [SiO4] tetrahedral may be a factor contributing to the smaller Stokes shift. Also, the larger radius of the Ba2+ ion in the K2Ba7Si16O40 host creates a larger interatomic distance between the Eu2+ ion and O2− ion. Thus, a little change in equilibrium distance in the 4f65d excited state would cause a small energy loss from the excited state to the ground state by the lattice vibration. As a result, a small Stokes shift was observed.13 Figure 5a depicts the PL intensity of K2Ba7(1−x)Si16O40:7xEu2+ as a function of the Eu2+ concentration (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) phosphor under the excitation at 440 nm. It can be seen clearly that the concentration of 0.06 mol is the critical concentration. The critical distance (Rc) for energy transfer among Eu2+ ions can be calculated from the following relationship given by Blass,23

Figure 5. (a) PL intensity of K2Ba7(1−x)Si16O40:7xEu2+ (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) phosphor under excitation at 440 nm. (b) The inset plots log[I /xEu 2 + ] vs log(xEu 2 + ) of K2Ba7(1−x)Si16O40:7xEu2+ (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) phosphor.

known that the resonant energy-transfer mechanism is either exchange interaction or electric multipolar interaction in solid compounds.23 Since the exchange interaction only fits the energy transfer of forbidden transitions, the involved wave function of energy transfer should have some overlap, and the typical critical distance is about 5 Å, the exchange interaction can be ruled out. Therefore, the energy-transfer mechanism is dominated by the multipole−multipole interaction, which can be easily deduced by Dexter theory as the following equation24,25 I = K[1 + β(x)Q /3 ]−1 x where Q = 6, 8, and 10 corresponding to electric dipole−dipole (d−d), dipole−quadrupole (d−q), and quadrupole−quadrupole (q−q) interactions, respectively. x presents the concentration of the activator and K and β are constants for a given excitation (λem = 440 nm) and host structure. For the purpose of obtaining a correct Q value in this case, the relationship between log(x) versus log(I /x) has been shown in Figure 5b. The Q/3 is calculated to be to −1.47. So the value of Q is found most approximately to be 6. The result indicates that the concentration quenching mechanism of the Eu2+ ion is dipole− dipole interaction.

⎛ 3V ⎞1/3 ⎟ R c = 2R = 2⎜ ⎝ 4πN ⎠

where V is the volume of the unit cell, xc is the critical concentration of the activator ion, and N represents the number of sites that the Eu2+ ion can occupy in per unit cell. For the K2Ba7Si16O40 host, N = 14, xc = 0.06, and V = 1999.80 Å3, Therefore, the Rc value obtained is about 16.57 Å. It is 4652

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Thermal stability is another important factor demanded for phosphors in practical application. Thermal relaxation of the lattice always makes the luminescence basically decline, so the phosphor should burden small intensity decreases below 150 °C for WLEDs. Figure 7 shows the intensity variations of the

Generally, luminescence decay curve analysis is a very important way to investigate the energy-transfer process. Figure 6 shows the room luminescence decay of Eu2+ ions in samples

Figure 6. Room luminescence decays of Eu2+ ions in samples K2Ba7(1−x)Si16O40:7xEu2+ (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14; λex = 440 nm, λem = 500 nm).

Figure 7. PL intensity of Sr2SiO4:Eu2+ and K2Ba7Si16O40:Eu2+ from room temperature to 230 °C (λex = 365 nm).

K2Ba7(1−x)Si16O40:7xEu2+ (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14; λex = 440 nm, λem = 500 nm). The decay profiles cannot be fitted by single-exponential function simply because of five luminescence centers in the host. Thus, the average decay time (τ*) should be determined by the formula as follows:26,27

commercial Sr2SiO4:Eu2+ and the K2Ba7Si16O40:Eu2+ from room temperature to 230 °C. The K2Ba7Si16O40:Eu2+ only shows less thermal quenching at 150 °C compared to Sr2SiO4:Eu2+, indicating its good suitability for WLEDs. Furthermore, a reduction of 35% of the initial intensity at 150 °C for the K2Ba6.93Si16O40:0.07Eu2+ is lower than that of 45% of the commercial YAG:Ce3+.29 The excellent thermal stability may result from the stiff structure that is built up on a corner-sharing [SiO4] tetrahedral network. The stiff structure always means the energy of the electrons in the excited state could not be easily released by lattice vibration, which can be reflected as smaller Stokes shift in a configurational coordinate diagram model. This model can also be used to describe the thermal quenching process. When the excited-state and the groundstate energy curves cross over at an energy level (ΔE), the electrons can be thermally activated to energy curves crossover from the relaxed excited state and then release the energy by generating lattice vibration, which quenches the luminescence.30 As a result, the value of the activation energy can be a criterion to estimate the thermal stability of the host lattice. The activation energy ΔE can be derived from a modified Arrhenius equation, I0 IT = ΔE 1 + c exp − kT

∞

τ* =

∫0 tI(t ) dt ∞

∫0 I(t ) dt

The calculated average lifetimes of the Eu2+ ions of K2Ba7(1−x)Si16O40:7xEu2+ (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) are 1.531, 1.394, 1.235, 1.132, 1.025, 0.927, 0.900, and 0.860 μs, respectively. As we know, the lifetime for Eu2+ ion is about microseconds (μs); the relatively shorter lifetime is due to the allowed electric-dipole 5d−4f transition caused by the poor spatial overlap between the 5d and 4f orbitals. The decay times are in accordance with the most frequent value of the normal Eu2+ emission in solids (∼1 μs). The lifetime decreases monotonically with the increase of the Eu2+ content, indicating an efficient energy transfer among Eu2+ ions. 3.4. Quantum Efficiency and Thermal Stability of K2Ba7Si16O40:Eu2+ Phosphor. Along with possessing an appropriate excitation wavelength, other factors are also required for phosphor application. One of the indispensable parameters is the quantum efficiency. Here we applied the integrated sphere method to measure absolute quantum yields (Φ) of the phosphors. The absolute quantum yields of K2Ba6.93Si16O40:0.07Eu2+ and K2Ba6.86Si16O40:0.14Eu2+ under the excitation of 440 nm are 61.9% and 53.4%, respectively. Since the quantum efficiency depends closely on the synthesis conditions, the particle size, morphology, and crystallinity of the phosphor,27,28 the higher quantum efficiency can be obtained by further optimization of the synthesis conditions. Therefore, the K2Ba7Si16O40:Eu2+ phosphor is a promising candidate for blue light WLEDs after optimization.

(

)

where I0 and IT correspond to the intensity of the initial and various temperatures, c is a constant for the same loss, ΔE is the activation energy related to this process, and k is the Boltzmann constant (8.629 × 10−5 eV K−1). Thus, the activation energy ΔE is calculated to be 0.133 eV. The larger value is an intuitive estimate for the thermal stability of K2Ba7Si16O40: Eu2+ phosphor.31,32 3.5. CIE Coordinates. The Commission International del’Eclairge chromaticity for emission from a 4% Eu-doped K2Ba7Si16O40 is calculated to have an index (x, y) = (0.233, 0.507) and the chromaticity point has been shown in the Figure 4653

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The Journal of Physical Chemistry C 8. It is worth noting that the line connecting the chromaticity points of a blue LED (λex = 440 nm) and any red phosphors

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REFERENCES

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crosses the Planckian locus. Therefore, the blue-excited K2Ba7Si16O40:Eu2+ phosphor has a great potential to be a candidate for WLEDs.

4. CONCLUSION In summary, we present a systematic study on the synthesis and crystal structure analysis of a novel K2Ba7Si16O40:Eu2+ phosphor and investigate their luminescence properties as a function of Eu2+ concentration. The Rietveld refinement of the crystal structure certifies that the Eu2+ ions are doped into the host successfully. Under the 440 nm blue light excitation, the phosphor shows efficiently a green emission band peaking at 500 nm and can compensate for the green region in the spectrum of WLEDs composed by a blue LED chip and greenred phosphors. The investigation reveals that the dipole−dipole interaction should be responsible for the concentration quenching of the Eu2+ ion. Furthermore, owing to the higher quantum efficiency, the better thermal stability, and the proper positions of the excitation and emission bands, this novel phosphor is a promising candidate for blue light converted WLEDs. ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work is financially supported by the National Natural Science Foundation of China (Grant21271167) and the Fund for Creative Research Groups (Grant 21221061), and the National Basic Research Program of China (973 Program, Grant 2014CB6438003).

Figure 8. CIE coordinate of K2Ba6.72Si16O40:0.28Eu2+ phosphor.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-431-85698041. Telephone: +86-431-85262798. Email: [email protected]. Notes

The authors declare no competing financial interest. 4654

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The Journal of Physical Chemistry C

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(31) Liu, C.; Xia, Z.; Lian, Z.; Zhou, J.; Yan, Q. J. Mater. Chem. C 2013, 1, 7139. (32) Lee, G.; Han, J. Y.; Im, W. B.; Cheong, S. H.; Young Jeon, D. Inorg. Chem. 2012, 51, 20.

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dx.doi.org/10.1021/jp500662a | J. Phys. Chem. C 2014, 118, 4649−4655