pubs.acs.org/Langmuir

Solvo-Hydrothermal Approach for the Shape-Selective Synthesis of

Vanadium Oxide Nanocrystals and Their Characterization

Thanh-Dinh Nguyen and Trong-On Do*

Department of Chemical Engineering, Laval University, Quebec G1K 7P4, Canada

Received December 10, 2008. Revised Manuscript Received January 24, 2009

A new solvo-hydrothermal method has been developed for the synthesis of uniform vanadium oxidenanocrystals (NCs) with various sizes and shapes in aliphatic amine/toluene/water using V(V) diperoxoalkylammonium complexes. The vanadium complex precursors were prepared from an ion exchange reactionof V(V) diperoxo gels and tetraalkylammonium bromide in the water-toluene mixture using H2O2 solution andcommercial bulk V2O5 powders as starting vanadium gel source. The obtained VO2 NC products werecharacterized by means of transmission electron microscopy (TEM), selected area electron diffraction (SAED),scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectra (XPS),Fourier transform infrared absorption spectroscopy (FTIR), thermogravimetric differential thermal analysis(TGA-DTA), and nitrogen adsorption/desorption (BET). The size and shape of NCs can be controlled bydifferent synthesis parameters such as water content, steric ligands of complexes, alkyl chain lengths of cappingaliphatic amines, as well as nature of solvent. Monodisperse vanadium oxide NCs with various sizes and shapes,nanospheres, nanocubes, nanorices, and nanorods, can be easily achieved. The possible mechanisms for theformation of vanadium complex precursors and vanadium oxide NCs as well as the shape evolution of NCs werealso discussed. The as-made vanadium oxide products exhibited the monoclinic rutile VO2 structure, which washowever converted to the orthorhombic V2O4.6 structure after calcination in air. The XPS results also revealedonly one V4+ state for the as-made sample; however, the coexistence of V5+ and V4+ states and two componentsof oxygen associated with OdV and O-V for the calcined samples on the vanadium oxide NC surface wereobserved. The surface chemical composition of both as-made and calcined samples were found to be VO2 andV2O5-x (x = 0.4), respectively. Our approach may provide a novel route for the extended synthesis to otherinorganic NCs.

1. Introduction

Shape-selective synthesis of inorganic nanocrystals (NCs) isparticularly attractive for exploring many of their uniquematerial properties with respect to their bulk counterparts,since they are potential building blocks for catalysis, advancedmaterials, and optoelectronic devices.1-6 For the past fewyears, various template-based methods including sol-gel,7

thermohydrolysis of organometallics,8 reverse micelle techni-que,9 and solvo-hydrothermal treatment,10,11 had been de-monstrated a success in superior shape and size control.Among them, a solvothermal approachof decomposingmetalcomplexes has been regarded as one of the most effective andeconomical routes,12 as it has the merits of a single step lowtemperature synthesis.

Vanadium oxides (V2O5-x) are of interest due to theirversatile redox activity and layered structures.13 They are akey technological material widely used in fields such aschemical sensing,14 actuators,15 high-energy lithium bat-teries,16 and electric field-effect transistors.17 In particular,V2O5-x NCs have high oxygen storage capacity (OSC) as aresult of the multiple valence state, so they are good candi-dates for applications in catalysis such as the selective reduc-tion of NOx by NH3

18 and oxidative dehydrogenation ofpropane.19 Moreover, supported vanadium oxides are foundto be selective catalysts in a number of catalytic reactions, forexample, oxidation of methanol, methane, and olefins, andoxidative convertion of o-xylene to phthalic anhydride.20-22

Further research interests will continue in the direction ofvanadium-based oxide NCs because of their advantages of

*To whom correspondence should be addressed. E-mail: Trong-On.Do@gch.ulaval.ca.

(1) Norris, D. J.; Efros, A. L.; Erwin, S. C. Science 2008, 319, 1776–1779.(2) Pachon, L.D.; Rothenberg, G.Appl. Organomet. Chem. 2008, 22, 288–

299.(3) Lu, A. H.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46,

1222–1244.(4) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science

2004, 303, 821–823.(5) Clfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350–2365.(6) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393–395.(7) Pinna, N.; Niederberger, M. Angew. Chem., Int. Ed. 2008, 47, 2–15.(8) Park, J.; An,K.;Hwang,Y.; Park, J.G.;Noh,H. J.;Kim, J.Y.; Park, J.

H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891–895.(9) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65–67.(10) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D.Nature 2005, 437, 121–124.(11) Xu, J.; Ge, J. P.; Li, Y. D. J. Phys. Chem. B 2006, 110, 2497–2501.(12) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389–1395. Peng,

Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343–3353.

(13) Shah, P. R.; Khader, M.M.; Vohs, J. M.; Gorte, R. J. J. Phys. Chem.C 2008, 112, 2613–2617.

(14) Livage, J. Chem. Mater. 1991, 3, 578–593.(15) Gu, G.; Schmid, M.; Chiu, P. W.; Minett, A.; Fraysse, J.; Kim, G. T.;

Roth, S.; Kozlov, M.; Munoz, E.; Baughman, R. H. Nat. Mater. 2003, 2,316–319.

(16) Poizot, P.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000,407, 496–499.

(17) Muster, J.; Kim, G. T.; Krstic, V.; Park, J. G.; Park, Y. W.; Roth, S.;Burghark, M. Adv. Mater. 2000, 12, 420–424.

(18) Chae, H.; Nam, I.; Ham, S.; Hong, S. Appl. Catal., B 2004, 53117–126.

(19) Barbero, B. P.; Cadus, L. E.; Hilaire, L. Appl. Catal., A 2003, 246,237–242.

(20) Blasco, T.; Lopez Nieto, J. M. Appl. Catal., A 1997, 157, 117–142.(21) Centi, G. Appl. Catal., A 1996, 147, 267–298.(22) Resini, C.; Milella, F.; Busca, G. Phys. Chem. Chem. Phys. 2000, 2,

2039–2045.

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© 2009 American Chemical Society

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being cheap and especially because they display high positiveredox potential for Li insertion.22-24

As a target for the shape-controlled NC synthesis, recentstudies in vanadium oxide NCs have focused on the develop-ment of synthetic approaches toward nanotubes, nanobelts,nanofibers, nanowires, nanorods, and so on, as well as theirshape-dependent properties. Up to date, some researchgroups25-27 reported the preparation of size-turnable mixed-valentVOxnanotubes via the aging andhydrothermal processof various vanadium sources including bulk V2O5 powders,vanadium(V) peroxo gels, and vanadium(V) triisopropoxidesusing aliphatic amines as structure-directing templates.Furthermore, the distance between the layers in the nanotubescan also be controlled by the alkyl chain length of thestructure-directing templates. Zhang et al.28 showed that thebelt-, olive-, petal-shaped VO2NCs could be synthesized withhigh concentrations of the reducing agent (specifically oxalicacid) through the hydrothermal route. Baughman et al.15

reported the synthesis of V2O5 nanofibers at room tempera-ture from ammonium metavanadate and acidic ion-exchangeresin in water. The resulting V2O5 nanofibers could deliverdramatically higher specific discharge capacities than micro-meter-sized V2O5 fibers. Recently, Whittingham’s group hasalso developed a method to produce vanadium oxide nanofi-bers with dimensions of less than 140 nmby coating oxides onpolylactide fibers.29 Park et al.30 synthesized single-crystallineVO2 nanowires with rectangular cross sections using a vaportransport method. Schlogl et al.31 presented a reverse micelletechnique to prepare V2O5 nanorods and nanowires from acolloidal self-assembly made of sodium bis(ethyl-2-hexyl)sulfosuccinate Na(AOT)/isooctane/H2O. Chen’s groupsynthesized vanadium oxide nanorods using such a reversemicelle technique.32 The aspect ratio (length/width) of thenanorods was found to be dependent on the water content inmicelle system. Cao et al.33 also synthesized V2O5 nanorodsarrays using electrophoretic deposition combined with tem-plating. Most of these synthetic methods are limited in theformation of 1-D nanostructures. Reports on the simpleroutes to synthesize different nanostructures with controlledsize and shape remain uncommon.

In this work, we demonstrate a simple modified solvother-mal method for the multigram scale synthesis of uniformvanadium oxide NCs using vanadium(V) diperoxo alkylam-moniumcomplexes in toluene or toluene/watermedium in thepresence of capping agents (e.g., aliphatic amines). The com-plex precursors were prepared using H2O2 solution andcommercial bulk V2O5 powder as vanadium source instead

of expensive organometallic compounds. Monodisperse va-nadium oxide NCs with different sizes and shapes includingnanospheres, nanocubes, nanorices, and nanorods can beachieved by the control of various reaction parameters, suchas water content, types of V(V) diperoxo alkylammoniumcomplexes, and alkyl chain length of capping agents in synth-esis mixture. This method can be extended to the synthesis ofother metal oxide NCs such as titanium oxide and niobiumoxide.

2. Experimental Section

2.1. Starting Materials. All chemicals were used as receivedwithout further purification. Vanadium pentaoxide (V2O5,99.6%), tetraoctylammonium bromide ([CH3(CH2)7]4NBr orTOA+, g98%), cetyl trimethylammonium bromide([CH3(CH2)15N(CH3)3]Br or CTA

+, 95%), tetrapropylammo-nium bromide ([CH3CH2CH2]4NBr or TPA+, 98%), oleyla-mine (C18H35NH2 or OM, tech. grade, 90%), hexadecylamine(C16H33NH2 or HA, tech. grade, 90%), dodecylamine(C12H25NH2 or DD, 98%), and octylamine (C8H17NH2 orOC, 97%) were purchased from Sigma-Aldrich. Hydrogenperoxide (H2O2, 30%) and all solvents used such as tolueneand absolute ethanol were of analytical grade and purchasedfrom Reagent ACS.

2.2. Synthesis of Vanadium Oxide Nanocrystals. 2.2.1.Preparation of Complex Precursors: Vanadium(V) DiperoxoAlkylammonium Complexes. Preparation of V(V) DiperoxoTetraoctylammonium Complexes, VO(O2)2(TOA). A total of0.14 g (0.77 mmol) of commercial bulk V2O5 powders wasdissolved in 20 mL of H2O2 solution (1.5%) and vigorouslystirred at room temperature for 30 min. V2O5 powders werecompletely reacted with dilute H2O2 solution to give a homo-geneous deep orange V(V) diperoxo aqueous solution (∼0.04mol/L, final pH ≈ 1.5-2.0) with a formula of [VO(O2)2]

-,according to the procedure reported by Zhang and Yu.34 Then,40 mL of toluene solution (0.05 mol/L) containing a cationicphase-transfer reagent ([CH3(CH2)7]4NBr or TOA+, 1.1 g) wasadded to the above solution, and the VO(O2)2

-/TOA+ molarratio was close to 1.0:2.5. The two-phasemixture was vigorouslystirred at room temperature. After 30min, [VO(O2)2]

- anions inthe aqueous phase were completely extracted into the toluenephase, and a deep orange toluene solution was observed. Sub-sequently, the upper deep orange supernatant toluene solution(40 mL) containing VO(O2)2(TOA) complexes was isolated.

Preparation of V(V) Diperoxo Cetyl Trimethylammo-nium, VO(O2)2(CTA), and V(V) Diperoxo Tetrapropylam-monium, VO(O2)2(TPA,) Complexes. The synthetic processof vanadate CTA compounds described by Livage et al.35 wasused with modification. In a typical preparation, 40 mL ofCTA+ aqueous solution (0.05 mol/L) (or TPA+) was addedto 20 mL of the above as-prepared V(V) diperoxo aqueoussolution, and the VO(O2)2

-/CTA+ (or TPA+) molar ratio wasalso close to 1.0:2.5. The resulting orange suspensive solutionwas stirred for 2 h, and then the water was slowly evaporated atroom temperature for 3 days, giving rise to orange solids of VO(O2)2(CTA) or VO(O2)2(TPA) complexes.

2.2.2. Synthesis of Vanadium Oxide Nanocrystals. Synth-esis of Vanadium Oxide Nanospheres from VO(O2)2(TOA)Precursors. Typically, 0.015 mol (5 mL) of oleylamine (OM)was added to 40 mL of the above as-prepared toluene solutioncontaining VO(O2)2(TOA) complexes under stirring. The reac-tion solution was transferred to a 60 mL Teflon-lined stainless-steel autoclave and heated at 180 �C for 5 h. The reaction systemwas cooled to room temperature using tap water. The vanadium

(23) Kim, G. T.; Muster, J.; Krstic, V.; Park, J. G.; Park, Y. W.; Roth, S.;Burghard, M. Appl. Phys. Lett. 2000, 76, 1875–1877.

(24) Karunakaran, C.; Senthilvelan, S. J. Colloid Interface Sci. 2005, 289,466–471.

(25) Tenne, R. Nature 2004, 431, 640–641.(26) Spahr, M. E.; Bitterli, P.; Nesper, R.; Muller, M.; Krumeich, F.;

Nissen, H. U. Angew. Chem., Int. Ed. 1998, 37, 1263–1265.(27) Corr, S. A.; Grossman, M.; Furman, J. D.; Melot, B. C.; Cheetham,

A. K.; Heier, K. R.; Seshadr, R. Chem. Mater. 2008, 20, 6396–6404.(28) Li, G.; Chao, K.; Peng, H.; Chen, K.; Zhang, Z. Inorg. Chem. 2007,

46, 5787–5790.(29) Lutta, S. T.; Dong,H.; Zavalij, P. Y.;Whittingham,M. S.Mater. Res.

Bull. 2005, 40, 383–393.(30) Guiton, B. S.; Gu, Q.; Prieto, A. L.; Gudiksen, M. S.; Park, H. J. Am.

Chem. Soc. 2005, 127, 498–499.(31) Pinna, N.;Willinger, M.;Weiss, K.; Urban, J.; Schlogl, R.Nano Lett.

2003, 3, 1131–1134.(32) Mai, L.; Guo, W.; Hu, B.; Jin, W.; Chen, W. J. Phys. Chem. C 2008,

112, 423–429.(33) Takahashi, K.; Limmer, S. J.; Wang, Y.; Cao, G. J. Phys. Chem. B

2004, 108, 9795–9800.

(34) Yu, L.; Zhang, X. Mater. Chem. Phys. 2004, 87, 168–172.(35) Bouhedja, L.; Steunou,N.;Maquet, J.; Livage, J. J. Solid State Chem.

2001, 162, 315–321.

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oxide products were easily precipitated by excess ethanol andredispersed preferably in nonpolar solvents (e.g., toluene, hex-ane, etc.). The precipitation-redispersion process was repeatedseveral times to purify the producing black OM-capped vana-dium oxide nanospheres.

Shape Transformation of Vanadium Oxide Nanospheresinto Nanorods by Controlling the Water Content. To inves-tigate the effect of water content on the shape transformation ofvanadium oxide nanospheres into nanorods during the solvo-hydrothermal synthesis, a fixed amount of distilled water(water/toluene ratio in volume, W/T = 0:40, 2:40, 8:40, and20:40) was added to the solution containing VO(O2)2(TOA)complexes. The autoclave was then sealed and heated at 180 �Cfor 5 h. The NC products recovered in the toluene phase werealso washed several times with ethanol to remove the residualoleylamine.

Size-Controlled Synthesis of Vanadium Oxide NCs(Including Nanospheres and Nanoplatelets) from VO(O2)2(CTA) and VO2(O2)2(TPA) Precursors. The aboveorange complex solids of as-prepared VO(O2)2(CTA) (0.80 g)complexes (or VO(O2)2(TPA), 0.65 g) were completely dissolvedin toluene medium (40 mL) under stirring, giving a clear orangesolution. To this solution, 0.015 mol (5 mL) of oleylaminewas then added. The reaction mixture was transferred to a60 mL Teflon-lined stainless-steel autoclave and heated at180 �C for 5 h.

Shape-Controlled Synthesis of Vanadium Oxide NCs byUsing Various Capping Aliphatic Amines and Ethanol Usedas the Synthesis Medium Instead of Toluene. The obtainedorange VO(O2)2(TOA) complex solids (1.1 g) (the above toluenesolution containing VO(O2)2(TOA) complexes after tolueneevaporation at room temperature) was added to 40 mL ofethanol solution containing other aliphatic amine compounds(0.015 mol) including OM, HA, DD, and OC (depending on thedesired particle shape) with stirring for 1 h at 70 �C. Theresulting suspension solution was transferred to a 60 mLTeflon-lined stainless-steel autoclave and heated at 180 �C for5 h. The reaction system was then cooled to room temperature,and the black products at the bottomof the vessel were collected.The colloidal alkyl amine capped vanadium oxide NCs wereredispersed in toluene and then precipitated by excess ethanol. Asimilar procedure was employed in the synthesis of othersamples as shown in Table 1.

2.3. Characterization. The particle sizes and morphologiesof the synthesized vanadium oxide NCs were determined at 120kV by using a JEOL JEM 1230 transmission electron micro-scope. Samples were prepared by placing a drop of a dilutetoluene dispersion of NCs onto a 200 mesh carbon coatedcopper grid and immediately evaporating at ambient tempera-ture. Scanning electron microscopy (SEM) analyses were car-ried out to see the overall morphology of the samples on theJEOL 6360 instrument with an accelerating voltage of 3 kV. Theaverage particle dimensions were determined by thesize distribution diagrams which were obtained from about

100-150 particles in representative transmission electron mi-croscopy (TEM) pictures of each sample.

The NC products were characterized on a Bruker SMARTAPEXII X-ray diffractometer operated at 1200 W power(40 kV, 30 mA) to generate Cu KR radiation (λ = 1.5418 A).The X-ray photoelectron spectra (XPS) were taken on a photo-electron spectrometer (KRATOSAXIS-ULTRA)with amono-chromatic X-ray source of Al KR. The operating conditions forrecording V 2p, O 1s, and C 1s high-resolution spectra were asfollows: 1486.6 eV and 225 W; pass energy of 160 eV with anoperating pressure of 10-9 Torr; and acquisition time of 5.75min, a pressed vanadium oxide pellet under an Ar+ bombard-ment. The V 2p and O 1s signals were measured simultaneouslyin one energy window. The peaks were deconvoluted by meansof standard CasaXPS software (v.2.3.13; product of CasaXPSSoftware Ltd.) in order to resolve the separate constituents afterbackground subtraction. Fourier tranform infrared (FTIR)spectra were measured with a FTS 45 infrared spectrophot-ometer with the KBr pellet technique. The thermal analyses ofthe as-made VO2NCs (∼5mg) were carried out at a heating rateof 10 �C/min under an air flux up to 650 �Cusing a Perkin-ElmerTGA thermogravimetric analyzer. Nitrogen adsorption/deso-rption isotherms were obtained by using a QuantachromeAutosorb-1 system. Prior to measurements, all samples weredegassed at a temperature of 200 �C for at least 2 h. The specificarea was calculated from the linear part of the Brunauer-Emmett-Teller (BET) equation (P/P0 ≈ 0.05-0.20). The poresize distributions were obtained from the analysis of the deso-rption branch of the isotherms using the Barret-Joyner-Halenda (BJH) model.

3. Results and Discussion

The vanadium oxide NCs were obtained using V(V) diper-oxo alkylammonium complexes in the presence of aliphaticamine compounds as capping agents through solvothermalor solvo-hydrothermal treatment at 180 �C for 5 h. Theirsize and shape were controlled by different reaction para-meters: (i) water content in the reactionmixture, (ii) types of V(V) diperoxo alkylammonium complexes (VO(O2)2(TOA),VO(O2)2(CTA), and VO(O2)2(TPA)), (iii) nature of solvents(toluene and ethanol), and (iv) nature of capping agents(oleylamine (OM), hexadecylamine (HA), dodecylamine(DD), and octylamine (OC)). The effect of these parameterson the size and shape of the final products was systematicallyexamined by TEM measurement. The vanadium oxide NCproducts obtained under different synthesis conditions arelisted in Table 1.

TheV(V) diperoxo gels were often obtained via the reactionbetween V2O5 powders and H2O2 solution.36 Other metal

Table 1. Synthesis Conditions, Shapes, and Sizes of the As-Made Capped Vanadium Oxide Nanocrystals

sample Figure precursors [mol]a surfactants [mol] solvents [mL]a T [�C] t [h] shape size [nm]

1 3a, 4b VO(O2)2(TOA) oleylamine W/T = 0:40 180 5 spherical 42 3b VO(O2)2(TOA) oleylamine W/T = 2:40 180 5 aggregated spherical 6-73 3c VO(O2)2(TOA) oleylamine W/T = 8:40 180 5 short rod ∼9 � 504 3d,e VO(O2)2(TOA) oleylamine W/T = 20:40 180 5 aligned rod 20 � 3005 4a VO(O2)2(TPA) oleylamine toluene 180 5 platelet 10-156 4c VO(O2)2(CTA) oleylamine toluene 180 5 spherical 57 5a VO(O2)2(TOA) oleylamine ethanol 180 5 cubic 58 5b VO(O2)2(TOA) hexadecylamine ethanol 180 5 rice ∼4 � 99 5c VO(O2)2(TOA) dodecylamine ethanol 180 5 thin rod ∼3 � 5010 5d VO(O2)2(TOA) octylamine ethanol 180 5 spherical 20-25

a [VO(O2)2]- = vanadium(V) diperoxo anion; TOA+ = tetraoctylammonium cation; CTA+ = cetyl trimethylammonium cation; TPA+ =

tetrapropylammonium cation; W/T = water/toluene ratio in volume.

(36) Dullberg, P. Z. Z. Phys. Chem. 1903, 45, 129.

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peroxo gels were developed for the wet synthesis of transitionmetal oxides and lamellar structures.34,37 However, in thecase of usingV(V)diperoxogels (e.g.,without organic ligands)as precursors, it is hard to control the size and shape ofmonodisperse NCs due to the heterogeneity in nonpolarmedia. To overcome this problem, in this study, we usedV(V) diperoxo alkylammonium complexes instead of V(V)diperoxo gels.

The V(V) diperoxo tetraoctylammonium complexes,VO(O2)2(TOA), were prepared from the two-phasesystem of V(V) diperoxo aqueous solution and toluenecontaining tetraoctylammonium ligands (TOA+) (Scheme 1).The formation of the complexes consists of two steps:

In the first step for the preparation of V(V) diperoxocompounds, the bulk V2O5 powders were completely dis-solved indiluteH2O2 aqueous solution to give the deeporange

solution, as seen in eq 1.34

V2O5 ðsolidÞ þ 4H2O2 f 2½VOðO2Þ2�-ðgelÞ þ 3H2O þ 2Hþ

ð1ÞThe possible geometry of the [VO(O2)2]

- anion is as theperoxide groups parallel each other, perpendicular to the oxooxygen and with the vanadium atom raised 0.5 A out of theplane.39 Coordination of a TOA+ ligand to the vanadiumcenter resulted in the formation of the pentagonal pyramidaloxodiperoxo tetraoctylammnium structure.38

In the second step, the [VO(O2)2]- anions incorporatedwith

the TOA+ ligands to yield the V(V) diperoxo tetraoctylam-monium complexes VO(O2)2(TOA) through the mass-trans-fer process of [VO(O2)2]

- from the aqueous phase into thetoluene phase. It is important to note that the change from aclear in-color to an orange color in the toluene phase, and

Scheme 1. (A) Photograph of a Sample Vial with the BiphasicMixture after the Transformation of Vanadium(V)DiperoxoAnions from theWater

Phase into the Toluene Phase; (B) Schematic Illustration of the Preparation of Vanadium(V) Diperoxo Tetraoctylammonium (VO(O)2(TOA))

Complexes (the Phase-Transfer Process Accompanied by the Corresponding Deep Orange Color One), Followed by the Formation of Alkyl Amine

Capped Vanadium Oxide NCs

(37) Do, T. O. Langmuir 1999, 15, 8561–8564.(38) Butler, A.; Clague, M. J.; Meister, G. E. Chem. Rev. 1994, 94

625–638. (39) Bortolini, O.; Conte, V. J. Inorg. Biochem. 2005, 99, 1549–1557.

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ArticleNguyen and Do

concomitantly from a deep orange in water phase to in-colorphase after this step, strongly suggests the presence of [VO(O2)2]

- anions in the toluene phase (see Scheme 1). The ionpairs ofVO(O2)2(TOA) complexes formeddue to electrostaticinteraction (VO(O2)2

-3 3 3TOA+) according to the equilibrium

are depicted by eq 2.

½VOðO2Þ2�-ðwaterÞ þ TOAþðtolueneÞfVOðO2Þ2ðTOAÞðtolueneÞ

ð2ÞThe formation of the VO(O2)2(TOA) complexes in the

toluene phase is further supported by the FTIR and thermo-gravimetric analysis (TGA) data. The FTIR spectrum of theVO(O2)2(TOA) complex solid (after toluene elimination) ex-hibits strong absorption FTIRbands characteristic of variousvibrationally active groups (Figure 1). The well defined IRbands at 2959, 2918, 2878, and 2852 cm-1 along with thebands at 733 and 720 cm-1 are characteristic of the symmetricand asymmetricmethyl andmethylene stretches of the TOA+

alkyl chains.40,41 An IR band is observed at 1455 cm-1 whichis attributed to the stretching vibrations of the ammoniumcation groups (tN+) of TOA+.42 The IR bands correspond-ing to stretches of the V(V) diperoxo appear in the 1000-500cm-1 region: 960 cm-1 for ν(VdO), 926 cm-1 for ν(Op-Op),and 542 cm-1 for ν(V-Op).

38,43 The TGA data for VO(O2)2(TOA) complex solid species (Figure 2) reveal a massloss of 34.8% in the temperature range 120-480 �C, which isattributed to the TOA+ decomposition and the combustionof organic groups. Furthermore, the complex species werecompletely transformed to vanadium oxides as the heatingtemperature reached 480 �C. A slight increase in mass ofapproximately 2% between 550 and 650 �C occurred due tothe oxidation of reduced vanadium oxides into V2O5.

35,44

These results confirm the formation of VO(O2)2(TOA) com-plexes through the transformation of [VO(O2)2]

- from aqu-eous phase into toluene phase in which TOA+ cations werecoordinated with [VO(O2)2]

- anions.

Effect of Water Content on the Formation of NCs. Underthe solvothermal treatments (e.g., in absence of water in thesynthesis solution), at high temperature (180 �C) in organicmedium, VO(O2)2(TOA) complex precursors are decom-posed and generate vanadium monomers and then vana-dium nuclei. They are capped by oleylamine molecules. Theresulting oleylamine-capped nuclei are hydrophobic andeasily dispersed within organic medium, which could growinto nanospheres through an Ostwald ripening process dur-ing the solvothermal synthesis. The probable reaction pro-cesses for the formation of capping vanadium oxideNCs canbe as follows:

It is important to note that a blue-black color was ob-served for all as-made vanadium oxide NCs, which corre-sponds to VO2, according to the X-ray diffraction (XRD)and XPS results (see below). Figure 3a shows a TEM imageof the samplewhichwas synthesized at 180 �C for 5 hwithoutwater in the toluene medium (sample 1 in Table 1). It can beclearly seen that the vanadium product consists of uniformquasispherical NCs with a diameter of 4 nm. The selectedarea electron diffraction (SAED) pattern taken by focusingthe electronic beam on a single vanadia particle (inset ofFigure 3a and c) composed of bright spots of respectivediffraction rings indicated the high crystallinity of vanadiumoxide nanospheres.

To verify the role of the TOA+ ligands for the formationof NCs, a similar experiment was carried out using V(V)diperoxo compound, [VO(O2)2]

-, as precursor (e.g., withoutTOA+ ligands) instead of VO(O2)2(TOA) complexes underthe same synthesis conditions. The results showed that nonanospheres were obtained but irregular particles closelyaggregated together were formed (S-Figure 1 in the Support-ing Information), suggesting the TOA ligands are a keyfactor for the formation of monodisperse vanadium oxidenanospheres in these synthesis conditions.

For the synthesis of vanadium oxide NCs startingfrom VO(O2)2(TOA) complex precursors, the significanteffect of water content on the size and shape of vanadiumoxide NCs was observed. In this study, the experiments

Figure 1. FTIR spectrum of the vanadium(V) diperoxo tetraocty-lammonium complex solid.

Figure 2. TGA curve of the vanadium(V) diperoxo tetraoctylam-monium complex solid.

(40) Brandstatter, M. K.; Riedmann, M. Microchim. Acta 1989, II173–184.

(41) Chalmers, J. M.; Bunn, A.; Willis, H. A.; Thorne, C.; Spragg, S.Microchim. Acta 1988, I, 287–290.

(42) Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20, 7117–7122.

(43) Kaliva, M.; Giannadaki, T.; Salifoglou, A. Inorg. Chem. 2001, 40,3711–3718.

(44) Carn, F.; Steunou,N.; Livage, J.; Colin, A.; Backov, R.Chem.Mater.2005, 17, 644–649.

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were carried out with different amounts of water addedin the synthesis solution (Table 1, samples 1-4), whilethe other synthesis conditions were kept constant. Keepingthe toluene volume unchanged (40 mL), the water/tolueneratio (W/T) in volume varied from 0:40 to 2:40, 8:40, and20:40. It was found that the shape evolution of vanadiumoxide NCs is a function of water content. Figure 3 showsrepresentative TEM images of these samples. In the absenceof water in the synthesis medium, only uniform quasisphe-rical NCs with an average size of 4 nm were obtained(Figure 3a). While the W/T value was as low as 2:40,aggregated nanoparticles beside some ununiformed smallnanorods were formed. Most of the aggregated nanoparti-cles formed pearl-chain-like structures; however, few iso-lated nanospheres were still observed (Figure 3b).

When the W/T value increased to 8:40, mostlyshort vanadium oxide nanorods (low aspect ratio) weregenerated. This indicates that with increasing the watercontent aggregated particles tend to align and form shortrods;47 however, the rod size distribution is broad with anaverage diameter of about 6-7 nm and length of 20-30 nm(Figure 3c). It is noted that even when the water content isincreased (up toW/T= 8:40), only one phase in the reaction

solution is observed. This is ascribed to the water/toluenemicroemulsion system owing to nanosized water dropletsdispersed in a continuous toluene medium and stabilized bysurfactant molecules at the W/T interface. The highly dis-persed water pools are ideal nanoreactors for producingmonodispersed nanoparticles and controlling shapes.48

However, when the W/T value increased to 20:40, the twowater/toluene phases were clearly separated in the synthesissolution. Interestingly, only the nanorod product wasformed in the toluene phase (Figure 3d), while no vanadiumoxide NCs were essentially observed in the water phase. Thevanadium product is composed of uniformly sized andshaped rods with ∼20 nm in width and 150-300 nm inlength. The high-magnification TEM image clearly demon-strates that the nanorods were quite smooth and straightalong their entire length (inset of Figure 3d). Furthermore,the rod diameters were quite uniform. The SAED patterntaken from a single vanadium oxide nanorod in the redsquare box (Figure 3d inset) reveals the single crystal natureof the nanorod, and it further confirmed that the nanorods’elongation axis was along the [101] direction. It means thatthe surface energy of the [101] face of the nanoparticles ishigher than that of other faces, and the growth by orientedaggregation could be dominant along the preferential direc-tion [101] at the W/T interface, so as to produce nanorods.Water in the synthesis mixture favors generally the forma-tion of nuclei and growth;without orwith lowwater contentsin the mixture, the chemical potential of monomer com-pounds is quite low and their motion is also low, which isdisadvantageous for the growth of nanorods. With highwater content (e.g., W/Tg 20:40), nucleation occurs at highspeed, forming a large quantity of tiny crystalline nuclei andconsequently leading to low monomer concentration in thebulk solution. Subsequently, the growth rate by orientedaggregation is dominant along the preferential direction (inthis work, along the [101] direction).

The increase in nanorod diameter from 7 to 20 nmwith theincrease of water content toW/T=20:40 could be explainedby a lateral aggregation of individual nanorods along thelongitudinal axis and further their fusion to form alignednanorods at the high water content in the mixture.47 So far,the syntheses were also carried out at higherW/T ratios (e.g.,W/T = 30/40 and 40/40). No obvious change in size andshape (constant aspect ratio) was observed. It implies thathigh water content (the W/T ratios g 20/40) plays a role tohave the two-phase system. In this two-phase system, bothnucleation and growth occur at the water/toluene interface.The formed nanorods at the interface can enter the organicphase and could stop growing.

Wealsobelieve that thedissolutionandgrowthof vanadiummonomers might also result in an overall growth of thenanorods. This suggests that particle size and shape can becontrolled by the water content in the reaction mixture underthese synthesis conditions.45-47 A schematic illustration of theinference from the TEMobservation demonstrating the trans-formation of nanospheres into nanorods is shown inScheme2.

In general, nanoporous materials have a large fraction ofatoms exposed at surfaces and access to reactant moleculesthat have attracted intensively for applications to catalysis.49

Figure 3. Effect of water content in the synthesis mixture on theshape transformation of nanospheres into nanorods. TEM imagesand corresponding SAED patterns of the as-made vanadium oxideNCs synthesized from V(V) diperoxo tetraoctylammonium com-plexes (0.04 mol/L) in oleylamine (5 mL) at 180 �C for 5 h withvarious water/toluene solvent ratios in volume (W/T): (a) W/T =0:40, (b) W/T = 2:40, (c) W/T = 8:40, and (d) W/T = 20:40. SEMimages of (e) the calcined vanadium oxide nanospheres and (f) thecalcined vanadium oxide nanorods (samples 1 and 4 in Table 1).

(45) Sigman, M. B.; Ghezelbash, J. A.; Hanrath, T.; Saunders, A. E.; Lee,F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050–16057.

(46) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L.Science 2000, 289, 751–754.

(47) Nguyen, T. D.; Mrabet, D.; Do, T. O. J. Phys. Chem. C 2008, 112,15226–15235.

(48) Biais, J.; Clin, B.; Laolanne, P. In Microemulsions: Structure andDynamics; Friberg, S. E., Bothorel, P., Eds.; CRC Press: Boca Raton, FL,1987; Chapter 1.

(49) Concepcion, P.;Knozinger,H.; LopezNieto, J.M.;Martinez-Arias J.Phys. Chem. B 2002, 106, 2574–2582.

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For the surfactant-capped nanoparticles, it is thereforenecessary to remove surfactants to generate porosity in thenanomaterial structure. Figure 3e and f represents SEMimages of vanadium oxide nanospheres (sample 1) andnanorods (sample 4) after calcination at 550 �C for 2 h. Asseen in Figure 3e for the vanadiumoxide nanosphere sample,the SEM image reveals that the spherical morphology withsome interlinked 3-D microspheres aggregated into biggerones was observed and inside each microsphere is mainlycomposed of monodisperse 4 nm sized nanospheres. How-ever, the SEM result of the calcined nanorod sample(Figure 3f) shows that the product consists of individualnanorods with ∼20 nm in diameter and 150-300 nm inlength, which is comparable with those observed in the high-magnification image (S-Figure 2 in the Supporting Informa-tion). As seen in S-Figure 2 (Supporting Information), eachnanorod is quite smooth and straight along the entire length.

Figure 4 shows the wide-angle XRD patterns of theas-made vanadium oxide nanosphere (sample 1 in Table 1)and nanorod (sample 4) samples before and after calcinationin air at 550 �C for 2 h. The X-ray pattern (Figure 4a)of the as-made samples 1 and 4 corresponds to monoclinicrutile-type VO2 (JCPDS card no. 44-0253, P42/mnm,

a = b = 0.4554 nm, c = 0.2856 nm).50,51 However, aftercalcination, the XRD patterns of the samples 1 and 4 exhibitthe orthorhombic V2O5-x phase (space group Pmmn, a =11.516 A, b = 3.566 A, c = 3.777 A; JCPDS card no. 41-1426).52 Furthermore, their color changed from blue-blackto yellow after calcination (see Figure 6d). This indicates thetransformation from the monoclinic rutile phase to theorthorhombic phase of these samples (Figure 4b,c). No otherphases were detected in the XRD patterns, suggesting thepure orthorhombic structure of the V2O5-x NCs. The par-ticle size of the calcined vanadium oxide nanosphere samplecalculated from the (001) reflections using the Debye-Scherrer equation is 5.2 nm,53 which is in agreement withthe sizes determined from the above TEM (Figures 3a and5b). It is important to note that the relative intensity of the(101) diffraction peak of the nanorods is much higher thanthat of the nanospheres, which indicates their anisotropicstructure and is in agreement with TEM and SAED results(Figure 3d).

To study the influence of the alkyl chain length of vana-dium complex ligands on the formation of vanadium oxideNCs, three different types of V(V) diperoxo alkylammoniumcomplexes with different alkyl chain ligands including VO(O2)2(TOA), VO(O2)2(CTA), andVO(O2)2(TPA) (see details

Scheme 2. Schematic Illustration of the Shape Transformation of VanadiumOxide NCs fromNanospheres into Nanorods As a Function ofWater

Content

Figure 4. XRDpatterns of vanadiumoxideNCsamples before andafter calcination: (a) as-made vanadium oxide nanospheres, (b)calcined vanadium oxide nanospheres, (c) as-made vanadium oxidenanorods, and (d) calcined vanadiumoxide nanorods (samples 1 and4 in Table 1).

Figure 5. XRD patterns of calcined vanadium oxide NCs synthe-sized from (a) VO(O2)2(TPA) and (b) VO(O2)2(CTA) (samples 5 and6 in Table 1).

(50) Corr, S. A.; Grossman, M.; Furman, J. D.; Melot, B. C.; Cheetham,A. K.; Heier, K. R.; Seshadri, R. Chem. Mater. 2008, 20, 6396–6404.

(51) Gui, Z.; Fan, R.; Mo, W.; Chen, X.; Yang, L.; Zhang, S.; Hu, Y.;Wang, Z.; Fan, W. Chem. Mater. 2002, 14, 5053–5056.

(52) Cao, A.M.; Hu, J. S.; Liang, H. P.; Wan, L. J.Angew. Chem., Int. Ed.2005, 44, 4391–4395.

(53) O’Dwyer, C.; Lavayen, V.; Fuenzalida, D.; Lozano, H.; SantaAna,M.; Benavente, E.; Gonzalez, G.; Torres, C. M. S. Small 2008, 4, 990–1000.

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in the Experimental Section) were used as starting precur-sors. The alkyl chain length is in the order TOA > CTA >TPA. The syntheses were conducted under the same condi-tions (samples 5 and 6, Table 1). According to their XRDpatterns, all the calcined products obtained from thesedifferent complex precursors have the orthorhombic phaseof V2O5-x (Figures 4 and 5). The particle sizes of calcinedsamples 5 and 6 calculated from the (001) reflections53 are∼12.0 and ∼6.5 nm, respectively. Representative TEMimages of these as-made NC products are also presented inFigure 6. The vanadium oxide sample prepared from VO(O2)2(TPA) is composed of irregular nanoplatelets withdiameters of 10-15 nm (Figure 6a). However, using VO(O2)2(TOA) and VO(O2)2(CTA), the quasispherical NCswere obtained with average diameters of ∼4 and ∼5 nm,respectively (Figure 6b and c), in agreement with the particlesizes obtained by XRD spectra. The corresponding SAEDpatterns (inset of Figure 6a-c) of these vanadium oxidesamples also composed of bright diffraction spots indicatedthe high crystallinity of vanadium oxide NCs. In general, theactivity of monomer complexes and, consequently, the sizeand shape of the resulting NCs can be affected by stericligands.54 Metal complex precursors with longer alkyl chainligands diffuse slower through the growth solution, and theyare also decomposed more slowly.55 In the case of TPA ascomplex ligands, their alkyl chain is shortest, the complexactivity was very high (rapid diffusion and decomposition),many nuclei were formed at the nucleation stage and aggre-gated immediately at the growth stage, and as a result a largerparticle size was obtained. However, longer alkyl chainligands such as TOA and CTA can balance the nucleationand growth rates, producing smaller NCs. Based on theTEM results, vanadium complexes with the longer alkylchains (TOA and CTA), as compared to that of TPA, were

decomposed more slowly to generate smaller nanoparticles.Therefore, it can be suggested that the particle sizes graduallyincreased from ∼4 to ∼5 and 10-15 nm due to the decreaseof alkyl chain length of complex ligands from TOA to CTAand then TPA.

It is important to note that, using this process, multigram-scale vanadium oxide NCs can be achieved in a single batch.For example, using VO(O2)2(CTA) complexes as precursors,about 6.8 g of oleylamine-capped quasispherical vanadiumoxideNCswas obtained in a single run (Figure 6d). The blue-black color of the products suggests that V5+ ions had beenreduced to V4+ by decomposing the precursors duringsynthesis,44 which is further confirmed by the XPS results(see below).

To understand the role of solvent medium on the particlesize and shape of the resulting vanadium oxide NCs duringthe synthesis,56 the experiment was carried out using VO(O2)2(TOA) complexes, and ethanol instead of toluene asreaction medium, while the other parameters were keptconstant. Uniform vanadium oxide nanocubes with sizes of∼5 nm in diameter were obtained (Figure 7a). This indicatesthe change in size and shape of vanadiumoxideNCs from∼4nm sized nanospheres (see Figure 3a) to ∼5 nm sizednanocubes when toluene was substituted by ethanol asmedium in the reaction system. However, the product yieldremained unchanged. This phenomenon may be due to thedifferent dielectric constant and polarity of ethanol andtoluene. The effect of different solvent media on the sizeand shape ofNCswill be systematically studied and reportedin a following paper.57,58

Furthermore, to gain further insights into the steric effectof the capping ligands on the size and shape of vanadiumoxide NCs, different alkyl chain lengths of the cappingaliphatic amines were used, such as oleylamine (OM, C18),

Figure 6. Effect of steric ligands of vanadium complexes on the sizeof vanadium oxide NCs. TEM images and corresponding SAEDpatterns of the VO2 NC samples prepared from different vanadiumcomplex precursors (e.g., various ligands): (a)VO(O2)2(TPA), (b) VO(O2)2(TOA), and (c)VO(O2)2(CTA).All of the syntheseswere carriedout under the same conditions as those for sample 1 in Table 1. (d)Photograph of multigrams of the as-made blue-black VO2 nano-sphere solids (6.8 g) and the calcined yellow V2O5 nanosphere solids(5.7 g) obtained in a single batch.

Figure 7. Shape evolution of vanadium oxide NCs as a function ofthe alkyl chain length of the capping aliphatic amine agents. TEMimages and corresponding SAED patterns of the as-made vanadiumoxide NC samples prepared from (a) oleylamine (OM, C18), (b)hexadecylamine (HD, C16), (c) dodecylamine (DD, C12), and (d)octylamine (OC,C8).All syntheseswere carriedusingVO(O2)2(TOA)complexes as vanadium precursors in ethanol medium.

(54) Hecht, S.; Frechet, M. J. Angew. Chem., Int. Ed. 2001, 40, 74–91.(55) Kim, J. I.; Lee, J. K. Adv. Funct. Mater. 2006, 16, 2077–2082.

(56) Aghabozorg,H.R.;Mousavi, R.; Askari, S.; Aghabozorg,H. J.NanoRes. 2007, 9, 497–500.

(57) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann:Woburn, MA, 1997.

(58) Wulff, G.; Zeitschrift, F. Beitr. Krystallogr. Mineral. 1901, 34, 449.

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hexadecylamine (HD, C16), dodecylamine (DD, C12), andoctylamine (OC, C8). The syntheses were carried out inethanol as reaction medium, and VO(O2)2(TOA) complexeswere used as vanadium precursors (Figure 7 and Table 1).The mass of VO(O2)2(TOA) complex solid (1.1 g) andvolume of ethanol (20 mL) were kept the same in allsyntheses (see details in the 2Experimental Section). It isfound that the vanadium oxide NCs exhibit various shapes,including cubic, ricelike particle, rod, and spherical particles,when the alkyl chain length of aliphatic amines is altered(Figure 7). In general, the effect of the capping alkyl chainlength is noticeable, as (i) it defines the lateral spacingbetween the particles and (ii) it is responsible for theiranisotropic growth into various shapes such as ricelike androd. Pimary NCs capped by shorter alkyl chain ligands aremore mobile and more reactive than those by longer ones,because they are less sterically impeded, which leads to fasterdiffusion and faster nucleation-growth kinetics,59 and ani-sotropic growth is favored.60

Our results revealed that when the long alkyl chain OM(C18) was used, uniform vanadium oxide nanocubes withsizes of 5 nm in diameter were produced (Figure 7a). How-ever, when HD (C16) was used instead of OM, elongatedparticles with a ricelike shape and a size of about 4 nm� 9 nmwere formed (Figure 7b). Furthermore, when HD was sub-stituted byDD (C12), the formation of thin nanorodswith anaverage size of 3 nm� 50 nm was observed (Figure 7c). Thissuggests that shorter alkyl chain ligands favor an anisotropicgrowth resulting in the formation of nanorods. However,with very short alkyl chain ligands (<C12) (in this work, weused OC (C8)), only spherical particles with a broad particlesize distribution and large diameter of 20-25 nm wereobserved (Figure 7d). The dispersity of the NCs preparedfrom HD, DD, and OC (shorter alkyl chain lengths com-pared to that ofOM)was not as good as that of theNCs fromOM, which could be due the smaller steric barrier.61

The corresponding SAED patterns (inset of Figure 7) ofcubic, ricelike, and spherical vanadium oxide particles ex-hibit a set of sharp diffraction spots which are characteristicof single crystalline of monoclinic rutile-type VO2 structure.

Furthermore, as seen in the insets of Figures 3d and 7c, forsamples 4 and 9, the SAED patterns of these samples arequite similar, indicating a good lattice matching between thetwo different samples. The same set of spots of sample 9compared to that of sample 4 suggests the thin rods alsoelongated along the [101] direction, which is consistent withtheir XRD results.

It is believed that the short alkyl chain length of aliphaticamines (<C12) as capping ligands could be hard to controlthe nucleation and growth because of much fast diffusionand growth kinetics. This can be concluded that the shapeevolution ofNCs can also dependon the alkyl chain length ofthe capping agents.

X-ray photoelectron spectroscopy (XPS) is a surface-specific technique that was also used to study the oxidationstates and the surface composition of the as-made andcalcined vanadium oxide nanosphere samples (sample 1before and after calcination). The survey XPS spectra ofthese samples in a wide energy range are shown in Figure 8.Besides carbons and nitrogens, no impurities were found.However, the intense C 1s and N 1s peaks of the as-madesamples as compared to those of the calcined sample can beassigned to the oleylamine ligand capped on the NC surface.The positions of the XPS peaks were corrected using the C 1score level taken at 285 eV as the binding energy (BE)reference. The XPS spectra of V 2p together with O 1s arepresented in Figure 9. The deconvolution data of the spectraare summarized in Table 2.62,63 Both the as-made andcalcined vanadium oxide nanosphere samples show the V2p states composed of two components originating from thespin-orbit splitting of V 2p3/2 and V 2p1/2, which arerestricted to a ratio of 1.8:1.0 and the separation energyΔ=8.3 eV. The V 2p3/2 peak has a small full width, 2.5-3.0eV, at half-maximum (fwhm), and the V 2p1/2 peak has alarger fwhm, 5.0-5.5 eV, due to multiplet effects.64 For theas-made sample 1, V 2p3/2 and V 2p1/2 peaks at 516.2 and

Figure 8. Survey XPS spectra of as-made vanadium oxide nano-spheres (a) before and (b) after calcination in air at 550 �C for 2 h(sample 1 in Table 1).

Figure 9. XPS spectra of V 2p andO 1s peaks and their deconvolu-tion of the as-made vanadium oxide nanosphere sample (a) beforeand (b) after calcination at 550 �C for 2 h.

(59) Lee, S. H.; Kim, Y. J.; Park, J. Chem. Mater. 2007, 19, 4670–4675.(60) Scher, E. C.; Manna, L.; Alivisatos, A. P. Philos. Trans. R. Soc.

London, Ser. A 2003, 361, 241–257.(61) Zhao,N.; Pan,D.;Nie,W.; Ji, X. J.Am.Chem. Soc. 2006, 128, 10118–

10124.

(62) Olivetti, E. A.; Kim, J. H.; Sadoway, D. R.; Asatekin, A.; Mayes, A.M. Chem. Mater. 2006, 18, 2828–2833.

(63) Barbero, B. P.; Cadus, L. E.; Hilaire, L. Appl. Catal., A 2003, 246,237–242.

(64) Guimaraes, J. L.; Abbate, M.; Betim, S. B.; Alves, M. C. M. J. AlloysCompd. 2003, 352, 16–20.

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524.1 eV, respectively, which are attributed to V4+ wereobserved. Non-XPS peaks corresponding to V5+ were de-tected (Figure 9a), suggesting that the surface chemicalcomposition in the as-made NC sample is VO2. However,the calcined sample 1 exhibits the presence of both V4+/V5+

oxidation states (Figure 9b). Both V 2p3/2 and V 2p1/2 peaksat 517.2 and 525.5 eV, respectively, which are characteristicof vanadium in the +5 oxidation state, are present. Simul-taneously, the V 2p3/2 and V 2p1/2 peaks at 516.2 and 524.1eV, respectively, corresponding to the V4+ oxidation statewere observed. From the relative areas of the V4+ and V5+

contributions, the surface vanadium was found to be ∼65%V5+ and∼35%V4+ for the calcined nanosphere sample (seeTable 2). It can be concluded that the change of color fromblue-black to yellow upon calcination in air could be due tothe transformation from the V4+ to V5+ oxidation state (seeFigure 6d).

Oxygen bonded to vanadium atoms in the as-made vana-dium oxide NC sample (sample 1) exhibits an O 1s peak(Figure 9a) related to O-V (530.7 eV).65 However, the O 1speak of this sample after calcination shows it is composed of

two components. The deconvolution peak at 530.4 eV wasassigned to the OdV groups, and another peak at a higherbinding energy of 531.0 eVwas attributed to theO-Vones,43

which are related to the oxygen in the crystal lattice ofvanadium oxides.66 Based on the corresponding areas ofthe vanadium and oxygen XPS peaks obtained by deconvo-lution, the molar ratio of V/O determined from the surfacechemical composition of the as-made and calcined sample 1was, respectively, (1.0):(2.0) and (1.0):(2.3). Thus, the che-mical surface formula is, respectively, VO2 and V2O4.6.

Figure 10 shows thermogravimetric differential thermalanalysis (TGA-DTA) curves of the OM-capped VO2 nano-sphere and OM-capped VO2 nanorod samples (samples 1and 4 in Table 1). For the nanosphere sample (Figure 11a),the steep weight loss (22%) appeared around 100-390 �C.However, the corresponding DTA curve shows two exother-mic peaks at 215 and 324 �C, which are related to thedecomposition and combustion of capping agents on theparticle surfaces during heating. A gradual mass gain of VO2

(∼2%) above 411 �C accompanied by a sharp endothermicpeak at 593 �C could be related to oxidation of the vanadiumdioxide framework from V4+ to V5+, as reported by Brand-statter andRiedmann.40 The total mass loss of nanorods was28%, quite similar to that of the nanospheres, 22% (samples1 and 2). However, for the VO2 nanorod sample(Figure 11b), the presence of an additional broad exothermicpeak at a higher temperature at 431 �C could be due tostrongly held surfactant molecules blocked between nanor-ods, in good agreement with our previously publishedresults.47

The FTIR spectrum of the as-made oleylamine (OM)-capped vanadiumoxide nanosphere sample (e.g., sample 1 inTable 1) is shown in Figure 11. The FTIR bands at 2925-2852 and 1625 cm-1 are assigned to the C-H and CdCstretching modes of alkyl chains in oleylamine, respectively.The band at 1458 cm-1 is attributed to theN-Hbending andN-C stretching modes of -NH2 groups in oleylaminecapping on vanadiumdioxides.67 Furthermore, the IR bandsat 620 and 927 cm-1 corresponding to the vibrations ofV-O-V and O-(V)3, respectively, along with an intenseFTIR band at 1022 cm-1 attributed to the V=O vibrationare characteristic of vanadium oxides.68 The presence ofthese FTIR bands suggests that oleylamine molecules are

Figure 10. TGA/DTA curves of (a) the as-made OM-capped va-nadium oxide nanospheres and (b) the as-made OM-capped VO2

nanorods (samples 1 and 4 in Table 1).

Figure 11. FTIR spectrum of the as-made OM-capped vanadiumoxide nanospheres (sample 1 in Table 1).

Table 2. XPS Binding Energies of O 2s, V 3p, V 3s, V 2p, and O 1s

Peaks in the As-Made Vanadium Oxide Nanosphere Sample before

and after Calcination at 550 �C for 2 h (Sample 1 in Table 1)

binding energy (eV)a

V 2p3/2 V 2p1/2 O 1s

sample 1 O 2s V 3p V 3s V4+ V5+ V4+ V5+ O- V4+ O- V5+

as-made 22.5 42.6 71.0 516.2 524.1 530.7calcined 22.5 42.6 71.0 516.2 517.2 524.1 525.5 531.0 530.4

aBE = 0.1 eV.

(65) Hecjingbottom, R; Linnett, J. W. Nature 1962, 194, 678.

(66) Chenakin, S. P.; Silvy, R. P.; Kruse, N. J. Phys. Chem. B 2005, 109,14611–14618.

(67) Gibot, P.; Laffont, L. J. Solid State Chem. 2007, 180, 695–701.(68) Chen, W.; Mai, L. Q.; Peng, J. F.; Xu, Q.; Zhu, Q. Y. J. Mater. Sci.

2004, 39, 2625–2627.

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bound to the surface of VO2 NCs. Similar results for theother aliphatic amine capped vanadium oxide NC sampleswere also observed (not shown).

The BET specific surface areas and pore size distributionsof the various samples after calcination were obtained bynitrogen adsorption/desorption isotherms and are summar-ized in Table 3 and S-Figure 3 in the Supporting Informa-tion. For the calcined nanosphere samples (samples 1 and 2in Table 1), the BET surface area was 70.5 and 68.5 m2/g forsample 1 and sample 2, respectively. The BET surface area ofsample 1 is higher than that of sample 2 due to its smallerparticle sizes.69 The average pore diameter with narrowmesopore size is around 16.5 and 19.5 nm for sample 1 andsample 2, respectively.

For samples 3 and 4 (Table 1) after calcination, the BETsurface area values are 42.0 and 32.0 m2/g with an averagepore size of 37.0 and 51.0 nm, respectively. The pore dia-meter of sample 4 is somewhat larger than that of sample 3due to the length difference of the rods. It is also noted thataverage pore diameters of the nanosphere samples (samples 1and 2) are smaller than those of the nanorod samples(samples 3 and 4) due to the compact structure.70 The highspecific surface areas of these nanoparticle samples aftercalcination can be explained by the particle surfaces beingprotected by the capping agent and the nanoparticle surfacebeing preserved during thermal procedures.66 The propertiesof such NCs could make them an interesting system forfuture studies in catalysis.

4. Conclusion

We have developed a new solvo-hydrothermal route tosynthesize monodisperse vanadium oxide NCs from vana-dium(V) diperoxo tetraalkylammonium complexes in alipha-tic amine/toluene or aliphatic amine/toluene/water.Commercial bulk V2O5 powders were used as a startingvanadium source. This approach is simple, economical, and

easily scaled up for products. The transformation of vana-diumoxide nanospheres into short nanorods aswell as alignednanorods can be controlled by water content in the synthesismixture. Furthermore, the shape evolution of vanadiumoxideproducts from nanospheres to nanoplatelets, nanocubes, andthen to nanorods was also controlled by the steric ligands ofvanadium complexes as well as the alkyl chain length ofcapping aliphatic amine agents in ethanol medium.

The obtained vanadium oxide NC products before calcina-tion are monoclinic rutile VO2 structure with black color,which is however converted to the orthorhombic V2O4.6

structure with yellow color upon the calcination procedure.The XPS results of these samples revealed that only V4+ wasobserved before calcination, while the coexistence of twovanadium oxidation states (V4+ and V5+) and two compo-nents of oxygen corresponding to OdV and O-V groups onthe vanadium pentaoxide NC surface was identified. Thesurface chemical composition of the vanadium oxide NCsample is VO2 and V2O5-x (x = 0.4) before and aftercalcination, respectively. The high surface area of the vana-dium oxide NC samples after calcination could be beneficialfor potential applications in many fields of advanced nano-technology, particularly in catalysis and chemical biologicalsensors. Furthermore, the synthetic method developed herecan be extended to the synthesis of other nanocrystallineoxides and is potentially useful for large-scale production ofnanomaterials.

Acknowledgment.Thisworkwas supported by theNaturalSciences and Engineering Research Council of Canada(NSERC) through a strategic grant. The authors would liketo thank Prof. S. Kaliaguine for stimulating discussions andcomments.

Supporting Information Available: TEM image of OM-capped VO2 NCs; high-magnification TEM image of cal-cined vanadium oxide nanorods; nitrogen adsorption/deso-rption isotherms and BJH pore size distributions ofvanadium oxide nanocrystal samples. This material is avail-able free of charge via the Internet at http://pubs.acs.org.

Table 3. BETSpecific Surface Area and Average PoreDiameters of the As-Made VanadiumOxide NCSamples after Calcination at 550 �C for 2 h

(Samples 1-4 in Table 1)

sample size and shape [nm] SBET [m2/g] VBJH [cm3/g] avg pore diameter [nm]

1 4 nm spheres 70.5 0.100 16.52 6-7 nm aggregated spheres 68.5 0.160 19.53 ∼9 � 50 nm rods 42.0 0.210 37.0

(69) Mrabet, D.; Zahedi-Niaki, M. H.; Do, T. O. J. Phys. Chem. C 2008,112, 7124–7129.

(70) Jaroniec, M.; Choma, J.; Gorka, J.; Zawislak, A.Chem.Mater. 2008,20, 1069–1075.

DOI: 10.1021/la804073a Langmuir 2009, 25(9),5322–53325332

Article Nguyen and Do

S 1

Solvo-hydrothermal Approach for the Shape-Selective Synthesis

of Vanadium Oxide Nanocrystals and Their Characterization

Thanh-Dinh Nguyen and Trong-On Do*

Department of Chemical Engineering, Laval University, Quebec G1K 7P4 Canada

To whom correspondence should be addressed. E-mail: Trong-On.Do@gch.ulaval.ca

SUPPORTING INFORMATION

S-Figure 1. TEM image of OM-capped VO2 NCs synthesized from the solvothermal

reaction of vanadium(V) diperoxo (without TOA+ ligands) in oleylamine/toluene at 180 oC for 5 h.

S 2

S-Figure 2. High-magnification TEM image of the calcined vanadium oxide nanorods

(calcined sample 4 in Table 1).

S 3

S-Figure 3. (a) Nitrogen adsorption (filled symbols) and desorption (open symbols)

isotherms and (b) BJH pore size distributions of vanadium oxide nanocrystal samples

after calcination at 550 oC for 2 h (samples 1-4 in Table 1).

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100 150 200

19.5 nm

0

50

100

150

200

250

0.0 0.2 0.4 0.6 0.8 1.0

dV/d

logD

(cm

3 /g)

Vol

ume

adso

rbed

(cm

3 /g)

Relative pressure (P/Po) Pore diameter (nm)

16.5 nm

51.0 nm

Sample 1

Sample 1

Sample 2

Sample 2

Sample 3

Sample 3

Sample 4

Sample 4

(a) (b)

37.0 nm