Communication pubs.acs.org/IC
Crystalline WS2 via Room Temperature, Solution-Phase Synthesis He Zhang and Adam S. Hock* Department of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, Illinois 60616, United States S Supporting Information *
believed that higher synthesis temperatures are necessary to achieve crystalline materials. Colloidal synthesis can also produce transition-metal dichalcogenides (TMDs) using metal halides, but high temperatures are used in existing processes.19,20 In the example20 of using WCl6 at 320 °C, the presence of hexamethyldisilazane in solution results in the semiconducting 2H phase where 1T is found when it is omitted. Organometallic approaches are limited to W(CO)6 and diphenyl diselenide, resulting in WSe2 at 330 °C with capping ligands21 or metal−organic chemical vapor deposition using W(CO)6 and dimethylselenium at 600−900 °C.22 However, metal chlorides and carbonyls do not provide the ability to tune the ligand reactivity of the precursor and control the synthesis by ligand modification. The slow addition of sulfur reagents is also required to achieve optimal WS2 material in many cases. In previous work, we have demonstrated 23 that coordinatively saturating ligands such as oCp (“open” cyclopentadienyl) can produce crystalline oxides at relatively low temperatures of ca. 120−150 °C. We hypothesized that the ability of oCp to adopt a variety of coordination modes24 might provide a surface metal atom with the ability to traverse the growing crystal faces and find a crystalline, stable binding site. Organometallic precursors have been used to produce nanoobjects25 by reduction with hydrogen gas, high temperature, high pressure, or thermally decomposed precursors. There are only a few examples of room temperature syntheses of nanomaterials using organometallic precursors26−28 that have so far appeared. Herein, we report a convenient method to prepare highly crystalline WS2 using bis(cyclopentadienyl)tungsten dihydride and sulfur at ambient temperature and pressure. The synthesis of bis(cyclopentadienyl)tungsten dihydride has been reported several times,29−31 and it was sublimed and recrystallized to ensure purity (NMR in Figures S1 and S2). Thermal stability tests of Cp2WH2 in C6D6 passed 100 °C for 24 h without observable decomposition when monitored by 1H NMR, and purified Cp2WH2 stored under a N2 atmosphere at room temperature for 1 month did not show observable decomposition. WS2 is simply prepared by adding a benzene solution of sulfur to a benzene solution of Cp2WH2. An orange precipitate appears in the original light-yellow solution within 1 min after mixing of the tungsten compound with S8. This is in contrast with the observed stability of Cp2WH2 toward protic reagents such as water (used in some synthetic procedures of Cp2WH2 during extraction).30 The stability of (iPrCp)2WH2 also leads to attempts to grow WO3 by atomic layer deposition with O2 plasma at 300 °C,17 and sulfurization of WO3 films using sulfur or
ABSTRACT: Crystalline tungsten disulfide (WS2) has been prepared from the reaction of bis(cyclopentadienyl) tungsten dihydride with sulfur at room temperature and ambient pressure in organic solvents. WS2 was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, resonance Raman spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy, and the resulting WS2 is highly crystalline by X-ray diffraction. The low-temperature synthesis is hypothesized to be a result of highly mobile surface W−Cp groups that are able to facilitate crystallization.
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t is generally accepted that high-temperature syntheses are necessary to achieve highly crystalline inorganic materials. Techniques such as the Czochralski process are used to produce large single crystals while crystalline thin films are prepared by molecular beam epitaxy1 and physical and chemical vapor deposition syntheses.2,3 The controlled syntheses of some crystalline nanoparticles are also often conducted at relatively high temperatures; however, the temperatures are lower than those of the above methods. Nanomaterial syntheses are interesting in the context of bulk crystalline material syntheses because they offer the opportunity to study how nucleation and growth mechanisms affect the crystallinity of the bulk materials. In our case, we have targeted the study of two-dimensional, layered chalcogenides because the growth should be favored in the layer direction if the edge chemistry results in in-plane growth controlled by the choice of the metal precursor ligand and synthesis chemistry. Layered chalcogenides such as WS2 are of ongoing interest in a variety of fields as a result of their many useful properties.4−6 These close-contact, van der Waals layered materials are traditionally known as lubricants7 and catalysts;8 however, more recently interest has arisen in their behavior as electronic materials9 and nanodevices.10,11 The synthesis of high-quality and large-grained materials is necessary to integrate layered chalcogenides with existing semiconductor fabrication paradigms. Previously, synthesis of WS2 was achieved only through high-temperature, harsh methods such as the reductive sulfurization of WO3 using H2S,12−14 W(CO)6 with S8 at high temperatures,15 solvothermal synthesis,16 and high-temperature vapor deposition.6,17 WS2 was prepared by atomic layer deposition as a monolayer by first depositing WO3, followed by sulfurization with S8 and characterization by Raman spectroscopy.17 Recently, WS2 was also made using tungsten and S8 at 500 °C and in a tellurium-assisted process.18 It is clear that existing methods use relatively high temperatures incompatible with semiconductor fabrication, and it is generally © XXXX American Chemical Society
Received: October 17, 2016
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DOI: 10.1021/acs.inorgchem.6b02490 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry H2S requires high temperature or plasma assistance.32 In contrast to previously reported syntheses, where heating is necessary, heating the reaction of Cp2WH2 and S8 to higher temperature resulted in carbon contaminations detected in energy-dispersive X-ray spectroscopy (EDX). Furthermore, the more sterically hindered Cp*2WH2 (Cp* = C5Me5) does not react with sulfur to produce WS2, even after long reaction times at room temperature. This is consistent with a metal-mediated mechanism for WS2 formation being blocked by the larger Cp* ligand. As a result, other substituted W−Cp complexes were not pursued. Scanning electron microscopy (SEM) images of WS2 show crystalline, layered structures stacked in an X shape (Figure 1a,b).
Figure 2. SEM images and EDX of the WS2 sample made in a dilute solution of toluene. The silicon peak is caused by a silicon base.
crystalline growth to have a larger average grain size. Along with a change in the crystal size, the morphology also changed to long and smooth spikes much bigger in size (around 200 μm diameter). Zoomed micrographs show (Figure 2a) that these spikes were stacked in layered structures. XRD also clearly shows the 2H structure of WS2. The sample was dispersed in ethanol without grinding to avoid solid-phase reactions and dried on a zero background mount. In the raw XRD spectrum (Figure S4), sulfur reflections are observed between the (002) and (004) reflections of WS2. Compared to the major reflection at about 14° (002), other reflections in XRD are at relatively lower intensities, indicating this crystalline solid is dominated by the (002) orientation, the flat planes. To distinguish the WS2 phase, the sulfur powder used in the experiment was also measured (Figure S5) and used as the background to subtract the sulfur phase found in the raw WS2 XRD spectrum. After subtraction of S8, only reflections of WS2 are seen in the new spectrum (Figure S6), indicating that the material obtained from this method is a mixture of WS2 and S8, with no detectable impurity phases. The sulfur stacked between these WS2 layers also had a different preferred orientation compared to the sulfur powder as the starting material shown by negative reflections in the difference spectrum. The larger-scale order also shows a layered material in the form of large-grain, crystalline WS2 in micrometer-scale flakes. The small excess also ensures total consumption of Cp2WH2. Raman spectra also agreed with the presence of WS2. Under 473 nm (Figure 3e), 532 nm (Figure 3f), and 633 nm (Figure 3g) laser excitation, the bulk features33 of WS2 can observed, with the presence of E12g and A1g peaks at around 350 and 420 cm−1 as the characteristic features to identify tungsten disulfide. Further Raman probing of the different thicknesses after sonication will be discussed in the following paragraph. To separate WS2 from intercalated sulfur, sonication was performed on the samples suspended in toluene. After 5 min (Figure 3a), the clusterlike WS2 broke into small pieces, and the layered structure was observed at the cracked edges. More explicitly, after 30 min (Figure 3b) of ultrasonic treatment, the layered structures were quite clearly exposed. The exfoliated thin sheets were examined by transmission electron microscopy
Figure 1. SEM images: (a) crystalline WS2 with S8 around made in benzene; (b) overview of a WS2 sample made in benzene; (c and d) WS2 made in toluene under the same conditions.
The powder X-ray diffraction (XRD) pattern (Supporting Information) and corresponding EDX obtained from the solid showed compositions of tungsten and sulfur in a ratio of 1:2.2 (Figure S3 and Table S1). The excess sulfur is entrained between the layered WS2 to some degree, providing a convenient means to exfoliate WS2 sheets by simple sonication in aromatic solvents (further details below). Carbon is below EDX detection limits and supports the clean nature of the WS2 synthesis. Changing the solvent to toluene results in smaller particle sizes, but they retain the similar X shape (Figure 1c,d). In one of the gas-phase synthesis studies,5 the initial nucleation shape showed a relationship with the crystals grown by observing grain boundaries. The similar X-shaped crystals indicate that the nuclei of these two experiments with different solvents have similar morphology but result in different sizes because toluene is more polar than benzene, making it harder to maintain continuous growth. Besides a comparison of different solvents, the effect of the precursor concentration on the synthesis was also investigated. Slower growth is known to provide higher crystalline quality, and as a result, the reaction run at 10× dilution produced very large flowerlike clusters with high crystallinity (Figure 2). EDX also shows a W/S ratio of 1:2.1 (Table S2), with the excess sulfur present as S8. Upon dilution, fewer nuclei formed at the initiation, and the low concentration and long reaction time allowed the B
DOI: 10.1021/acs.inorgchem.6b02490 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
In conclusion, we demonstrated that crystalline WS2 can be synthesized at room temperature in solution from the organometallic precursor Cp2WH2. The resulting WS2 is of high purity and simply exfoliated to solution sheets by sonication in organic solvents. This result shows the flexibility of organometallic reagents for the controlled synthesis of nanomaterials and could direct a new way of fabricating two-dimensional materials such as TMDs, an emerging attractive field. Further studies on the utilization of other organometallic precursors are ongoing.
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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.6b02490. Experimental procedures, NMR of the precursor, SEM of thin sheets and other controls, EDX data, XRD data, Raman spectra, and EXAFS data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
He Zhang: 0000-0001-9028-9263 Adam S. Hock: 0000-0003-1440-1473 Notes
The authors declare no competing financial interest.
Figure 3. Top: SEM images of (a) WS2 and (b) after sonication for 5 min (a) and 30 min (b). Middle (c and d): TEM images of WS2 exfoliated by sonication. Bottom: Raman spectra under 473 nm (e), 532 nm (f), and 633 nm (g) laser excitation.
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ACKNOWLEDGMENTS This work was supported, in part, by the Center for Low Energy Systems Technology, one of six centers of STARnet, a Semiconductor Research Corp. program sponsored by MARCO and DARPA. We also thank Illinois Institute of Technology (IIT) and the IIT Center for Synchrotron Radiation Research and Instrumentation for support. The authors gratefully thank Prof. Carlo U. Segre for assistance with SEM and XRD characterizations. Dr. Guanghui Zhang provided his thoughtful advice and helped with XAS measurements.
(TEM). The thin sheets observed in SEM (Figure S7) can be seen clearly in TEM (Figure 3c,d), and these thin sheets also observed as layer upon layer and folded sheets from edges in a range of these sheets are shown in the TEM images. Raman spectra under 514 nm laser excitation (Figure S8) also showed a decrease in thickness after sonication, corresponding to the WS2 sheets observed in electron microscopy. Of note, no surfactant was added nor was an attempt made to redisperse the suspension of WS2 sheets as a monolayer on the TEM grid. The brighter areas (Figure S8c,e) observed under the microscope have higher intensities at peaks around 350 cm−1 (Figure S8d,f) compared to the bulk area (Figure S8a,b). The increase of second-order Raman peaks is represented as the 2LA(M) mode,33 with decreasing numbers of layers. The use of a further stabilization of the ligands to atomically prepare thin materials, as demonstrated by Tan et al.,34 was not pursued. X-ray absorption spectroscopy (XAS) was also used to measure the WS2 sample, and in the R space of extended X-ray absorption fine structure (EXAFS), only one peak at around 2 Å indicates only sulfur as the neighboring atoms next to tungsten (Figure S9 and a fitting summary in Table S3). The sulfur between layered structures allowed exfoliation to be reached easily by sonication. As we observed in a comparison of the SEM images of WS2 grown in benzene and toluene and at different reaction concentrations, variations of the reaction conditions do have some effects on the morphology of the crystalline solids;35,36 however, a detailed study of the effect of the solvent and concentration on the morphology is beyond the scope of this study.
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REFERENCES
(1) Joyce, B. A. Molecular Beam Epitaxy. Rep. Prog. Phys. 1985, 48, 1637−1637. (2) Helmersson, U.; Lattemann, M.; Bohlmark, J.; Ehiasarian, A. P.; Gudmundsson, J. T. Ionized Physical Vapor Deposition (IPVD): A Review of Technology and Applications. Thin Solid Films 2006, 513, 1− 24. (3) Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L. Crystallinity of Inorganic Films Grown by Atomic Layer Deposition: Overview and General Trends. J. Appl. Phys. 2013, 113, 021301− 021301. (4) Elías, A. L.; Perea-López, N.; Castro-Beltrán, A.; Berkdemir, A.; Lv, R.; Feng, S.; Long, A. D.; Hayashi, T.; Kim, Y. A.; Endo, M.; Gutiérrez, H. R.; Pradhan, N. R.; Balicas, L.; Mallouk, T. E.; López-Urías, F.; Terrones, H.; Terrones, M. Controlled Synthesis and Transfer of LargeArea WS2 Sheets: From Single Layer to Few Layers. ACS Nano 2013, 7, 5235−5242. (5) Zhang, Y.; Zhang, Y.; Ji, Q.; Ju, J.; Yuan, H.; Shi, J.; Gao, T.; Ma, D.; Liu, M.; Chen, Y.; Song, X.; Hwang, H. Y.; Cui, Y.; Liu, Z. Controlled Growth of High Quality Monolayer WS2 Layers on Sapphire and Imaging its Grain Boundary. ACS Nano 2013, 7, 8963−8971. (6) Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; López-Urías, F.; Crespi, V. H.; Terrones, H.; Terrones,
C
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Communication
Inorganic Chemistry M. Extraordinary Room-Temperature Photoluminescence in Triangular WS2Monolayers. Nano Lett. 2013, 13, 3447−3454. (7) Tenne, R.; Rapoport, L.; Bilik, Y.; Feldman, Y.; Homyonfer, M.; Cohen, S. R. Hollow Nanoparticles of WS2 as Potential Solid-State Lubricants. Nature 1997, 387, 791−793. (8) Sobczynski, A.; Yildiz, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T.; Webber, S. E.; White, J. M. Tungsten Disulfide: A Novel Hydrogen Evolution Catalyst for Water Decomposition. J. Phys. Chem. 1988, 92, 2311−2315. (9) Deng, Y.; Luo, Z.; Conrad, N. J.; Liu, H.; Gong, Y.; Najmaei, S.; Ajayan, P. M.; Lou, J.; Xu, X.; Ye, P. D. Black Phosphorus−Monolayer MoS2 van der Waals Heterojunction p−n Diode. ACS Nano 2014, 8, 8292−8299. (10) Perea-López, N.; Elías, A. L.; Berkdemir, A.; Castro-Beltran, A.; Gutiérrez, H. R.; Feng, S.; Lv, R.; Hayashi, T.; López-Urías, F.; Ghosh, S.; Muchharla, B.; Talapatra, S.; Terrones, H.; Terrones, M. Photosensor Device Based on Few-Layered WS2 Films. Adv. Funct. Mater. 2013, 23, 5511−5517. (11) Liu, H.; Si, M.; Deng, Y.; Neal, A. T.; Du, Y.; Najmaei, S.; Ajayan, P. M.; Lou, J.; Ye, P. D. Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: An Insight into Current Flow across Schottky Barriers. ACS Nano 2014, 8, 1031−1038. (12) Seifert, G.; Köhler, T.; Tenne, R. Stability of Metal Chalcogenide Nanotubes. J. Phys. Chem. B 2002, 106, 2497−2501. (13) Whitby, R. L. D.; Hsu, W. K.; Fearon, P. K.; Billingham, N. C.; Maurin, I.; Kroto, H. W.; Walton, D. R. M.; Boothroyd, C. B.; Firth, S.; Clark, R. J. H.; Collison, D. Multiwalled Carbon Nanotubes Coated with Tungsten Disulfide. Chem. Mater. 2002, 14, 2209−2217. (14) Shi, Y.; Wan, Y.; Liu, R.; Tu, B.; Zhao, D. Synthesis of Highly Ordered Mesoporous Crystalline WS2 and MoS2 via a HighTemperature Reductive Sulfuration Route. J. Am. Chem. Soc. 2007, 129, 9522−9531. (15) Lee, D.-W.; Tolochko, O. V.; Turaev, F. R.; Kim, D.; Kim, B.-K. Synthesis and Characterization of WS2 Nanoparticles by Chemical Vapor Condensation. J. Nanosci. Nanotechnol. 2010, 10, 349−354. (16) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296−7299. (17) Song, J.-G.; Park, J.; Lee, W.; Choi, T.; Jung, H.; Lee, C. W.; Hwang, S.-H.; Myoung, J. M.; Jung, J.-H.; Kim, S.-H.; Lansalot-Matras, C.; Kim, H. Layer-Controlled, Wafer-Scale, and Conformal Synthesis of Tungsten Disulfide Nanosheets Using Atomic Layer Deposition. ACS Nano 2013, 7, 11333−11340. (18) Gong, Y.; Lin, Z.; Ye, G.; Shi, G.; Feng, S.; Lei, Y.; Elías, A. L.; Perea-Lopez, N.; Vajtai, R.; Terrones, H.; Liu, Z.; Terrones, M.; Ajayan, P. M. Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2Monolayers. ACS Nano 2015, 9, 11658−11666. (19) Jeong, S.; Yoo, D.; Jang, J.; Kim, M.; Cheon, J. Well-Defined Colloidal 2-D Layered Transition-Metal Chalcogenide Nanocrystals via Generalized Synthetic Protocols. J. Am. Chem. Soc. 2012, 134, 18233− 18236. (20) Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. Colloidal Synthesis of 1T-WS2 and 2H-WS2 Nanosheets: Applications for Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136, 14121−14127. (21) Jung, W.; Lee, S.; Yoo, D.; Jeong, S.; Miró, P.; Kuc, A.; Heine, T.; Cheon, J. Colloidal Synthesis of Single-Layer MSe2 (M = Mo, W) Nanosheets via Anisotropic Solution-Phase Growth Approach. J. Am. Chem. Soc. 2015, 137, 7266−7269. (22) Eichfeld, S. M.; Hossain, L.; Lin, Y.-C.; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Kim, M. J.; Mayer, T. S.; Redwing, J. M.; Robinson, J. A. Highly Scalable, Atomically Thin WSe2 Grown via Metal−Organic Chemical Vapor Deposition. ACS Nano 2015, 9, 2080− 2087. (23) Riha, S. C.; Racowski, J. M.; Lanci, M. P.; Klug, J. A.; Hock, A. S.; Martinson, A. B. F. Phase Discrimination through Oxidant Selection in Low-Temperature Atomic Layer Deposition of Crystalline Iron Oxides. Langmuir 2013, 29, 3439−3445.
(24) O’Connor, J. M.; Casey, C. P. Ring-Slippage Chemistry of Transition Metal Cyclopentadienyl and Indenyl Complexes. Chem. Rev. 1987, 87, 307−318. (25) Cormary, B.; Dumestre, F.; Liakakos, N.; Soulantica, K.; Chaudret, B. Organometallic Precursors of Nano-Objects, a Critical View. Dalt. Trans. 2013, 42, 12546−12553. (26) Soulantica, K.; Maisonnat, A.; Fromen, M. C.; Casanove, M. J.; Lecante, P.; Chaudret, B. Synthesis and Self-Assembly of Monodisperse Indium Nanoparticles Prepared from the Organometallic Precursor [In (η5-C5H5)]. Angew. Chem. 2001, 113, 462−465. (27) Wang, W.; Germanenko, I.; El-Shall, M. S. Room-Temperature Synthesis and Characterization of Nanocrystalline CdS, ZnS, and CdxZn1-xS. Chem. Mater. 2002, 14, 3028−3033. (28) Monge, M.; Kahn, M. L.; Maisonnat, A.; Chaudret, B. RoomTemperature Organometallic Synthesis of Soluble and Crystalline ZnO Nanoparticles of Controlled Size and Shape. Angew. Chem., Int. Ed. 2003, 42, 5321−5324. (29) Chen, K. S.; Kleinberg, J.; Landgrebe, J. A. Insertion and Substitution Reactions of Dihalocarbene Precursors with bis(h5Cyclopentadienyl)tungsten Dihydride. Inorg. Chem. 1973, 12, 2826− 2828. (30) Persson, C.; Andersson, C. WCl4(DME): A Facile Entry into Bis(cyclopentadienyl) Complexes of Tungsten(IV). Synthesis of Bis(cyclopentadienyl)tungsten Dihydride. Organometallics 1993, 12, 2370−2371. (31) Jones, W. D.; Chin, R. M.; Crane, T. W.; Baruch, D. M. CarbonSulfur Bond Cleavage in Thiophene by Group 6 Metallocenes. Organometallics 1994, 13, 4448−4452. (32) Morrish, R.; Haak, T.; Wolden, C. A. Low-Temperature Synthesis of n-Type WS2 Thin Films via H2S Plasma Sulfurization of WO3. Chem. Mater. 2014, 26, 3986−3992. (33) Berkdemir, A.; Gutiérrez, H. R.; Botello-Méndez, A. R.; PereaLópez, N.; Elías, A. L.; Chia, C.-I.; Wang, B.; Crespi, V. H.; López-Urías, F.; Charlier, J.-C.; Terrones, H.; Terrones, M. Identification of Individual and Few Layers of WS2 using Raman Spectroscopy. Sci. Rep. 2013, 3, 1755−1755. (34) Tan, S. M.; Sofer, Z.; Luxa, J.; Pumera, M. Aromatic-Exfoliated Transition Metal Dichalcogenides: Implications for Inherent Electrochemistry and Hydrogen Evolution. ACS Catal. 2016, 6, 4594−4607. (35) Kahn, M. L.; Monge, M.; Colliere, V.; Senocq, F.; Maisonnat, A.; Chaudret, B. Size-and Shape-Control of Crystalline Zinc Oxide Nanoparticles: A New Organometallic Synthetic Method. Adv. Funct. Mater. 2005, 15, 458−468. (36) Niederberger, M. Nonaqueous Sol−Gel Routes to Metal Oxide Nanoparticles. Acc. Chem. Res. 2007, 40, 793−800.
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DOI: 10.1021/acs.inorgchem.6b02490 Inorg. Chem. XXXX, XXX, XXX−XXX
