Enhanced Hydrogen Storage Properties of Mg–Ti–V Nanocomposite

Article pubs.acs.org/JPCC

Enhanced Hydrogen Storage Properties of Mg−Ti−V Nanocomposite at Moderate Temperatures Tong Liu,*,† Chunguang Chen,† Hui Wang,† and Ying Wu‡ †

Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China ‡ China Iron & Steel Research Institute Group, Advanced Technology & Materials Co., Ltd., No. 76 Xueyuannanlu, Haidian District, Beijing 100081, China S Supporting Information *

ABSTRACT: With the purpose of improving the hydrogen sorption kinetics of Mg at moderate temperatures, the Mg− TiH1.971−VH2 nanocomposite is synthesized by means of the hydrogen plasma−metal reaction (HPMR) approach. The nanocomposite is consequently hydrogenated and dehydrogenated at 673 K to synthesize Mg−9.6 wt % Ti−2.9 wt % V nanocomposite. The Mg Nanoparticles (NPs) in hexagonal shape range from 30 to 300 nm. The spherical Ti and V NPs of about 23 nm are uniformly distributed on the surfaces of the Mg NPs. The Mg−Ti−V nanocomposite quickly absorbs 4.7 wt % H2 within 5 min at 473 K and 2.5 wt % H2 within 10 min at 373 K. The apparent activation energies for hydrogenation and dehydrogenation are 29.2 and 73.8 kJ mol−1, respectively. During the hydrogenation/dehydrogenation cycle, the Ti and V NPs restrain the growth of Mg NPs. The dehydrogenation of TiH1.971 to Ti is interpreted as the catalytic impact of V NPs. The improved sorption kinetics at moderate temperatures and the reduced activation energy derive from the nanostructure of Mg and the synergic catalytic impact of Ti and V NPs. (Cu, Fe, Mn, V, Ni, Co, Ti, and Nb)8−13 or their oxides (Fe2O3, V2O5, and Nb2O5)14,15 can significantly increase the sorption rate of Mg without apparently decreasing its hydrogen storage capacity. High energy ball milling (HEBM) is the most common method to disperse additives in Mg. For example, the ball-milled Mg−Co alloys were able to absorb 2.1 mass % H2 at 323 K.12 It was reported that the Mg particles catalyzed by Nb2O5 can uptake H2 at 298 K and dehydrogenate at 473 K.14 As a catalyst, the addition of Ti can remarkably improve the hydrogen storage properties of Mg at moderate temperatures (99.9%) ingot of 15 g and Ti50V50 master alloy of 10 g in the presence of Ar (0.05 MPa) and H2 (0.05 MPa). During the processing, the discharge current was kept at 260 A. The NPs were collected after the passivation by a mixture of argon and air. The Mg NPs were also synthesized by the HPMR method at the same condition as the Mg−Ti−V nanocomposite. A Sieverts-type apparatus with a reactor chamber volume of 60 mL was used to measure the hydrogen desorption and absorption properties. Before the measurement, the as-prepared Mg-based nanocomposite of 100 mg was heated to 673 K to experience one hydrogen absorption/desorption cycle in

3. RESULTS AND DISCUSSION 3.1. Particle Features. The TEM image in Figure 1a presents the Mg-based nanocomposite synthesized by the HPMR approach. The NPs can be classified into two types: the big ones and the small ones. Like the Mg ultrafine particles synthesized by HPMR,28 the big particles are of hexagonal shape. The high-resolution TEM (HR-TEM) image of one small particle on the big particle is shown in Figure 1b. The interplanar spacing of the big particle is measured to be 2.448 22420

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are due to the slow evaporation rate of Ti and V than Mg in the HPMR process. The XRD pattern of the as-prepared nanocomposite in Figure 2a shows that the Mg-based nanocomposite prepared by HPMR is mainly made of α-Mg (hcp) and a small amount of TiH1.971 and VH2, which agrees with the TEM result (Figure 1). No diffraction peaks of MgH2 are found. For Mg, Ti, and V, whether hydride nanoparticles can be produced during HPMR process depends on their hydrogenation kinetics and thermodynamics. The hydrogenantion rate of Mg strongly depends on the hydrogen pressure. Our previous study has proved that low H2 pressure of 0.05 MPa and the high cooling rate during HPMR prevent the formation of magnesium hydride.28 It is known that V and Ti in microsize are able to uptake hydrogen at much lower temperature and hydrogen pressure than Mg. Under arc plasma, Ti and V clusters are unable to react with hydrogen thermodynamically. It is suggested that during the cooling process in HPMR the extremely large surface area and high activity of the Ti/V nanoparticles (23 nm) enable them to uptake H2 and form hydride even at 0.05 MPa of hydrogen. It should be pointed out that MgO is always detected in the XRD analysis of the Mg particles prepared by HPMR (see Figure S1c). However, no diffraction peaks of MgO are detectable in the XRD pattern of the as-prepared Mg−Ti−V nanocomposite, indicating that the pyrophoricity of Mg is effectively suppressed by Ti and V. It is shown in Figure 2b that after one hydrogenation/dehydrogenation cycle at 673 K MgH2 dehydrogenates fully into α-Mg, and VH2 transforms to V. It has been reported that TiH1.971 decomposes to TiH1.5 during the dehydrogenation process at around 673 K.30−32 Similarly, in our previous study on Mg−Ti nanocomposite, the TiH1.971 NPs of about 13 nm only partly dehydrogenate to TiH1.5 at 673 K, and no Ti phase can be found.33 In this work, surprisingly, TiH1.971 completely dehydrogenates to Ti (see Figure 2b). The only difference between the Mg−Ti−V system and the Mg−Ti system is the existence of V NPs. In this regard, we propose that the full dehydrogenation of TiH1.971 can be interpreted as the catalytic function of V NPs. Since the hydrogenation/ dehydrogenation of V takes place at moderate condition, the addition of V NPs of high activity can accelerate the dehydrogenation behavior of TiH1.971 NPs. Therefore, the initial state of the Mg−Ti−V nanocomposite before the hydrogenation measurement is the Mg−9.6 wt % Ti−2.9 wt % V nanocomposite. It can be seen in Figure 2c that after the hydrogenation of the Mg-based nanocomposite at 673 K Mg

Figure 2. XRD patterns of the as-prepared Mg-based nanocomposite by HPMR (a), the samples obtained after hydrogenation/dehydrogenation at 673 K (b), and after the absorption under 4 MPa hydrogen pressure at 673 K (c).

Å, corresponding to the Mg (101), 2.452 Å. This demonstrates that the big particles are Mg. The big particles range from 30 to 300 nm and have a mean particle size of 155 nm (see the inset in Figure 1a). Compared with the Mg NPs of about 310 nm (see Figure S1a,b) on average prepared by HPMR, the addition of Ti and V greatly reduces the particle size of Mg. The small spherical particles of about 23 nm are dispersed on these big Mg particles. The interplanar spacing of the small particle in Figure 1b is measured to be 2.561 Å, corresponding to the TiH1.971 (111), 2.563 Å. The HR-TEM image of another small particle exhibits a interplanar spacing of 2.462 Å, which belongs to the (111) of VH2, 2.459 Å (see Figure 1c). It is known that the dehydrogenation process is time-related, and the full decomposition of hydride takes a certain length of time. Although VH2 hydride is not stable during the TEM observation, most of VH2 NPs probably do not dehydrognate fully. This has also been proved in our previous study of the Mg−V nanocomposite, where VH2 NPs have been also observed by TEM.25 It is suggested that the uniformly distributed small spherical Ti/V hydrides NPs on Mg effectively prevent the direct contact of Mg particles. In this regard, they would prevent the growth or agglomeration of Mg particles. The EDS result in Figure 1d displays that the Mg−Ti−V nanocomposite contains 9.6 wt % Ti and 2.9 wt % V. Therefore, the as-prepared nanocomposite synthesized by HPMR is Mg−9.9 wt % TiH1.971−3.0 wt %VH2. The lower Ti and V contents in the as-prepared NPs than in the raw alloys

Figure 3. TEM bright-field image of Mg−Ti−V nanocomposite after the hydrogenation/dehydrogenation process at 673 K (a), HR-TEM image of one small nanoparticle on the big particle (b), and another small nanoparticle (c). 22421

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Figure 4. Hydrogenation curves under 4 MPa hydrogen pressure (a) and dehydrogenation curves under 100 Pa hydrogen pressure (b) at different temperatures, absorption (c), and desorption (d) plots of ln k vs 1000/T of the Mg−Ti−V nanocomposite.

zone. It is worth to note that the lattice fringe spacing of the big particle is 2.778 Å, corresponding to the (100) plane of α-Mg, and no fringe of MgH2 can be observed. This proves that MgH2 dehydrogenates completely into α-Mg. As for the small particle, the evaluated interplanar spacing is 2.339 Å, corresponding to standard data of Ti (110) (JCPDS 44-1288). Figure 3c displays the HR-TEM image of another selected zone of the NPs. The lattice fringes with a spacing of 2.145 Å belongs to (110) plane of V. The TEM results conincide with the XRD analysis (Figure 2b). Figure S2 presents the TEM and HR-TEM images of the Mg-based nanocomposite hydrogenated at 673 K. The NPs are still polyhedral in shape, ranging from 60 to 280 nm, of 164 nm on average. The small particles disperse uniformly on the large particles. It is found that the lattice fringe of the big particle in Figure S2b corresponds to MgH2 (101) with a spacing of 2.510 Å. The fine black particle is determined as TiH1.971 and keeps similar particle size of about 25 nm as in the as-prepared nanocomposite. On the basis of the analysis above, the following equations explain the HPMR, dehydrogenation, and hydrogenation processes of the Mg−Ti−V nanocomposite.

Figure 5. Illustration of the synergic catalytic effects of the Ti and V NPs.

transforms into MgH2, and Ti and V change into TiH1.971 and VH2. The particle morphology and size of the Mg-based nanocomposite do not change apparently after the hydrogenation/dehydrogenation at 673 K (see the TEM image in Figure 3a). The big particles possess polyhedral shape, and the particle size ranges from 50 to 290 nm, about 160 nm on average. Figure 3b displays the HR-TEM image of one selected

Mg + Ti + V + H 2 → Mg + TiH1.971 + VH 2

(1)

Figure 6. P−C isotherm curves at 623, 648, and 673 K (a) and Van’t Hoff plots (b) for the Mg−Ti−V nanocomposite. 22422

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MgH2 + TiH1.971 + VH 2 → Mg + Ti + V + H 2

(2)

Mg + Ti + V + H 2 → MgH2 + TiH1.971 + VH 2

(3)

than that of the Mg−V NPs of 71.2 kJ mol−1.25 This value is also far below the 25 nm Mg particles of 122 kJ mol−136 and the Mg NPs prepared by HPMR of 123.8 kJ mol−1 shown in Figure S4a. The dehydrogenation activation energy of the nanocomposite is evaluated to be 73.8 kJ mol−1 by the same method (see Figure 4d), smaller than the Mg−V NPs of 119.4 kJ mol−1 25 and the Mg NPs of 127.7 kJ mol−1 (see Figure S4b). This clearly demonstrates that the Ti and V NPs efficiently improve the hydrogenation/dehydrogenation kinetics of Mg. The enhanced hydrogenation kinetics of Mg can be explained from the following two factors: the nanostructure and the synergic catalytic impact of Ti and V NPs. First, the nanostructure of Mg plays an important role in the sorption process. Compared with microscale particles, the NPs mean short diffusion distances and also provide more dissociation sites, leading to the enhanced sorption kinetics. Second, the Ti and V NPs are also critical to improve the hydriding properties of the nanocomposite, especially at moderate temperatures. It is believed that pure Mg without catalytic additives does not have sufficient ability to dissociate hydrogen molecules40 and shows poor hydrogenation performance at moderate temperatures (Figure S3a). In the present work, the Ti and V NPs act as catalysts to initially adsorb and spill over hydrogen to Mg surfaces and decrease the activation energy to dissociate H2.41,42 Since Ti and V NPs do not cover the surface of Mg completely, the dissociated hydrogen can diffuse directly into Mg without diffusing through Ti and V nanoparticles. The schematic mechanism for the catalytic effects of Ti and V NPs is illustrated in Figure 5. It is also demonstrated that the diffusion of hydrogen into Mg is retarded by the MgO layer.43 The Ti and V NPs outside the Mg particles decrease the oxidation of Mg and facilitate the hydrogenation of Mg. They also restrain the Mg NPs from growing during hydrogenation/dehydrogenation cycles. More importantly, Ti and V NPs can further absorb a certain amount of hydrogen to form TiH1.971 and VH2 under moderate condition, which enables them to dissociate hydrogen easily. Thus, the synergic catalytic effects of the Ti and V NPs are more efficient to accelerate the hydrogenation rate of Mg than V alone. In this regard, the reduced activation energy of the nanocomposite, to a large extent, is attributed to the coexistence of Ti and V NPs. We further investigate the thermodynamics of the Mg−Ti−V nanocomposite. The P−C−T plots at 623, 648, and 673 K of the Mg−Ti−V nanocomposite are shown in Figure 6a. Since there are only small amounts of Ti and V in the nanocomposite, each plot exhibits only one flat plateau. The equilibrium pressures for the hydrogenation are 0.55, 1.11, and 1.62 MPa at 623, 648, and 673 K, respectively. The pressures of the dehydrogenation plateaus are 0.44, 0.84, and 1.30 MPa 623, 648, and 673 K. Then, the Van’t Hoff plots (ln P vs 1/T) for both hydrogenation and dehydrogenation are shown in Figure 6b. The equation obtained from fitting line of the hydrogenation is ln(P) = −9.09/T + 18.64 with a goodness of linear fit of 0.956. The hydrogenation enthalpy (ΔHab) of the nanocomposite is determined as −75.6 kJ mol−1, which is similar to the standard value for MgH2 (∼−75 kJ mol−1 H2).41,44,45 The dehydrogenation equation derived from Figure 6b is ln(P) = −9.10/T + 18.42. The evaluated dehydrogenation enthalpy (ΔHde) is 75.7 kJ mol−1, and the goodness of linear fit is 0.984. This implies that the thermodynamics of the hydrogenation/dehyhdrogenation is not changed apparently by Ti and V NPs, even if they strongly accelerate the sorption rate. The Mg−Ti−V nanocomposite with high storage capacity

3.2. Hydrogen Storage Properties. The microscale particles of Mg only uptake 1.5 wt % H2 for 2 h at 673 K even if they are activated for several times.34,35 The hydrogenation curves at various temperatures under 4 MPa are plotted in Figure 4a. It is observed that the hydrogenation performance of the Mg−Ti−V nanocomposite at moderate temperatures is remarkable. The nanocomposite can quickly uptake 2.5 wt % H2 in 10 min at 373 K, superior to the Mg−V NPs that only absorbs less than 1 wt % H2 in 60 min at the same temperature.25 This indicates that the synergic catalytic effect of Ti and V NPs is more effective than V alone. It is also found in Figure 4a that these curves exhibit very high initial hydrogenation speeds. For example, the nanocomposite can quickly absorb 4.8 wt % H2 within 5 min at 473 K. The Mg NPs synthesized by the HPMR method can only uptake 2.0 wt % at 473 K in 60 min (see Figure S3a). The sorption rate of the Mg−Ti−V nanocomposite is also faster than that of Mg NPs of 38 nm, which can absorb nearly 2.5 wt % in 25 min at 493 K.36 The theoretical hydrogenation gravimetric capacity of the Mg− 9.6 wt % Ti−2.9 wt % V nanocomposite is 6.7 wt %. The storage capacity of the Mg−Ti−V nanocomposite reaches 5.4, 6.0, and 6.7 wt % at 573, 623, and 673 K, respectively. Compared with the Mg−polymer nanocomposite with a storage capacity of 4 wt % as a result of high polymer content,21 the Ti and V NPs not only prevent Mg NPs from growing during hydrogenation/dehydrogenation but also offer the Mg−Ti−V nanocomposite a large storage capacity of 6.7 wt %. The theoretical hydrogenation gravimetric capacity of Mg can be as high as 7.6 wt %. Because of the partial oxidation, however, the storage capacity of the Mg NPs produced by HPMR only reaches 6.4 wt % at 673 K (Figure S3a). For the dehydrogenation process, the reaction rate of the Mg−Ti−V nanocomposite also increases between 473 and 673 K (see Figure 4b). It should be noted that at 623 K the Mg−Ti−V nanocomposite is able to desorb 5.0 wt % H2 in 5 min, better than the Mg−V system which desorbs 4.6 wt % H2 at 623 K.25 The Mg−Ti−V nanocomposite releases 4.0 wt % H2 at 573 K in 5 min, better than the Mg NPs which only desorb 0.8 wt % H2 in 30 min at the same temperature (see Figure S3b). The dehydrogenation capacity of the nanocomposite also increases with the growing temperature and reaches 4.1, 5.1, and 5.2 wt % at 573, 623, and 673 K, respectively. The JMAK (Johnson−Mehl−Avrami−Kolmogorov) model is often used to express the hydrogen sorption kinetics (see eq 4)37 ln[− ln(1 − α)] = η ln k + η ln t

(4)

where k is the reaction rate constant, η is the Avrami exponent of reaction order, and α is the fraction transformed at time t. For each temperature, the slope η and the intercept η ln(k) can be obtained from the plot of ln[−ln(1 − α)] vs ln(t). Then, the Arrhenius equation is used to calculate the hydrogenation activation energy:38,39 k = A exp( −Ea /RT )

(5) −1

−1

where R is the gas constant (8.314 472 J mol K ), T represents the absolute temperature, and Ea is the activation energy. The hydrogenation activation energy of the nanocomposite calculated from Figure 4c is 29.2 kJ mol−1, smaller 22423

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(7) Li, W. Y.; Li, C. S.; Ma, H.; Chen, J. Magnesium Nanowires: Enhanced Kinetics for Hydrogen Absorption and Desorption. J. Am. Chem. Soc. 2007, 129, 6710−6711. (8) Liang, G.; Huot, J.; Boily, S.; Van Neste, A.; Schulz, R. Catalytic Effect of Transition Metals on Hydrogen Sorption in Nanocrystalline Ball Milled MgH2-Tm (Tm = Ti, V, Mn, Fe and Ni) Systems. J. Alloys Compd. 1999, 292, 247−252. (9) Rousselot, S.; Bichat, M.-P.; Guay, D.; Roué, L. Structure and Electrochemical Behaviour of Metastable Mg50Ti50 Alloy Prepared by Ball Milling. J. Power Sources. 2008, 175, 621−624. (10) Krishnan, G.; Palasantzas, G.; Kooi, B. J. Influence of Ti on the Formation and Stability of Gas-Phase Mg Nanoparticles. Appl. Phys. Lett. 2010, 97, 261912.1−261912.3. (11) Liang, G.; Huot, J.; Boily, S.; Van Neste, A.; Schulz, R. Hydrogen Storage Properties of the Mechanically Milled MgH2-V Nanocomposite. J. Alloys Compd. 1999, 291, 295−299. (12) Shao, H. Y.; Matsuda, J.; Li, H. Y.; Akiba, E.; Jain, A.; Ichikawa, T.; Kojima, Y. Phase and Morphology Evolution Study of Ball Milled Mg-Co Hydrogen Storage Alloys. Int. J. Hydrogen Energy. 2013, 38, 7070−7076. (13) Jin, S.; Shim, J.; Ahn, J.; Cho, Y. W.; Yi, K. Improvement in Hydrogen Sorption Kinetics of MgH2 with Nb Hydride Catalyst. Acta Mater. 2007, 15, 5073−5079. (14) Hanada, N.; Ichikawa, T.; Hino, S.; Fujii, H. Remarkable Improvement of Hydrogen Sorption Kinetics in Magnesium Catalyzed with Nb2O5. J. Alloys Compd. 2006, 420, 46−49. (15) Song, M. Y.; Bobet, J. L.; Darriet, B. Improvement in Hydrogen Sorption Properties of Mg by Reactive Mechanical Grinding with Cr2O3, Al2O3 and CeO2. J. Alloys Compd. 2002, 340, 256−262. (16) Shao, H.; Felderhoff, M.; Schüth, F. Hydrogen Storage Properties of Nanostructured MgH2/TiH2 Composite Prepared by Ball Milling under High Hydrogen Pressure. Int. J. Hydrogen Energy 2011, 36, 10828−10833. (17) Cummings, D. L.; Powers, G. J. The Storage of Hydrogen as Metal Hydrides. Ind. Eng. Chem. Process Des. Dev. 1974, 13, 182−192. (18) Lu, J.; Choi, Y. J.; Fang, Z. Z.; Sohn, H. Y.; Rönnebro, E. Hydrogenation of Nanocrystalline Mg at Room Temperature in the Presence of TiH2. J. Am. Chem. Soc. 2010, 132, 6616−6617. (19) Shao, H. Y.; Xin, G. B.; Zheng, J.; Li, X. G.; Akiba, E. Nanotechnology in Mg-Based Materials for Hydrogen Storage. Nano Energy 2012, 1, 590−601. (20) Wagemans, R. W. P.; Van Lenthe, J. H.; De Jongh, P. E.; Van Dillen, A. J.; De Jong, K. P. Hydrogen Storage in Magnesium Clusters: Quantum Chemical Study. J. Am. Chem. Soc. 2005, 127, 16675−16680. (21) Jongh, P. E.; Adelhelm, P. Nanosizing and Nanoconfinement: New Strategies Towards Meeting Hydrogen Storage Goals. ChemSusChem 2010, 3, 1332−1348. (22) Jeon, K. J.; Moon, H. R.; Ruminski, A. M.; Jiang, B.; Kisielowski, C.; Bardhan, R.; Urban, J. J. Air-Stable Magnesium Nanocomposites Provide Rapid and High-Capacity Hydrogen Storage without Using Heavy-Metal Catalysts. Nat. Mater. 2011, 10, 286−290. (23) Aguey-Zinsou, K. F.; Ares-Fernández, J. R. Synthesis of Colloidal Magnesium: A Near Room Temperature Store for Hydrogen. Chem. Mater. 2008, 20, 376−378. (24) Bardhan, R.; Ruminski, A. M.; Brand, A.; Urban, J. J. Magnesium Nanocrystal-Polymer Composites: A New Platform for Designer Hydrogen Storage Materials. Energy Environ. Sci. 2011, 4, 4882−4895. (25) Liu, T.; Zhang, T. W.; Qin, C. G.; Zhu, M.; Li, X. G. Improved Hydrogen Storage Properties of Mg-V Nanoparticles Prepared by Hydrogen Plasma-Metal Reaction. J. Power Sources. 2011, 196, 9599− 9604. (26) Liu, T.; Qin, C. G.; Zhang, T. W.; Cao, Y. R.; Zhu, M.; Li, X. G. Synthesis of Mg@Mg17Al12 Ultrafine Particles with Superior Hydrogen Storage Properties by Hydrogen Plasma-Metal Reaction. J. Mater. Chem. 2012, 22, 19831−19838. (27) Liu, T.; Zhang, T. W.; Zhu, M.; Qin, C. G. Synthesis and Structures of Al-Ti Nanoparticles by Hydrogen Plasma-Metal Reaction. J. Nanopart. Res. 2012, 14, 738−745.

and rapid sorption rate at moderate temperatures is promising for the hydrogen storage application.

4. CONCLUSIONS The Mg−9.6 wt % Ti−2.9 wt % V nanocomposite was successfully prepared by HPMR and the hydrogenation/ dehydrogenation process. The Ti and V NPs of 23 nm were uniformly dispersed on the surfaces of the Mg NPs of 30−300 nm, and they prevented the Mg NPs from growing during the hydrogenation/dehydrogenation. The Mg−Ti−V nanocomposite can quickly uptake 2.5 wt % H2 in 10 min at 373 K. The calculated hydrogenation/dehydrogenation activation energies were 29.2 and 73.8 kJ mol−1, respectively. The enhanced sorption rate and storage capacity at moderate temperatures are attributed to the synergic impacts of the Ti and V NPs and the nanostructure of Mg.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86 10 8231 6192; e-mail [email protected] (T.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by MOST of China (No. 2013CB035503), China Program of Magnetic Confinement Fusion under Grant 2012GB102006, the Aeronautical Science Foundation of China (No. 2011ZF51065), the National Natural Science Foundation of China (Grant 51371056), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. We also acknowledge the technical support from the Center for Instrumental Analysis and Research of Beihang University.

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp5061073 | J. Phys. Chem. C 2014, 118, 22419−22425