A Method for Fabrication of Pyramid-Shaped TiO2 Nanoparticles with a High {001} FacetPercentage

Yahya Alivov* and Z. Y. Fan†

Nano Tech Center and Department of Electrical and Computer Engineering, Texas Tech UniVersity,P.O. Box 43102, Lubbock, Texas 79409-3102

ReceiVed: June 2, 2009; ReVised Manuscript ReceiVed: June 19, 2009

A new method is demonstrated for fabricating TiO2 nanoparticles. Transformation of TiO2 nanotubes totruncated pyramid-like-shaped nanoparticles with a high percentage of photocatalytically active {001} facetshas been observed in titanium dioxide (TiO2) ordered nanotube arrays after thermal annealing in ambientfluorine. It was found that the nanotube-nanoparticle transformation resulted from catalytic reaction of fluorineions (F-) from the electrolyte residues in long nanotubes grown by anodization in ethylene glycol + NH4Felectrolyte. This transformation occurs at annealing conditions when evaporation of electrolyte residues innanotubes is slow enough to remain until 500 °C, when this transition starts. The size of the formed TiO2

nanoparticles depends on the fluorine concentration and can be controlled from 20 to 500 nm. The crystalproperties of nanoparticle layers are superior compared to those of nanotube arrays. This work demonstratesa simple method for producing high-quality anatase TiO2 nanoparticles and nanoparticle-based electrodeswith a high ratio of reactive {001} facets.

Nanostructured titanium dioxide (TiO2) has attracted greatattention recently1-5 due to its unique properties that make themof considerable scientific interest and practical importance. TiO2,including both nanoparticles (NPs) and nanotubes (NTs), hasbeen used for dye-sensitized solar cells (DSSC), water splittingfor hydrogen generation, photocatalysis for purification of airand water, and bio- and chemical sensors.4,6,7 To facilitatevectorial electron transport for efficient photoconversion, theorganized nanotube structure has its advantages. Two mask-free techniques, hydrothermal synthesis by NaOH treatment ofTiO2 nanoparticles with subsequent acid washing3,8 and elec-trochemical anodic oxidation of titanium metal foil in fluorinatedelectrolyte,4,9,10 have been widely reported for the synthesis ofTiO2-based nanotubes. It has been explained11 that in thehydrothermal synthesis by treatment with NaOH, some Ti-Obonds are broken, leading to the formation of lamellar sodium-containing titanate fragments, which would undergo Na+

exchange with H+ in the post-treatment acid washing. The ionexchange results in the variation of surface charge and peeling-off of the individual layers of titanate (nanosheets). The scrollingof the sheets forms individual nanotubes. In the anodizationmethod, first discovered in 1991 by Zwilling et al.,12 organizednanotube arrays formed under the balance of TiO2 formationand chemical dissolution in the electrolyte during electrochemi-cal oxidation of Ti sheets.4 It should be pointed out that thehydrothermally formed product is a random mixture of indi-vidual nanotubes, while the electrochemically formed productis a regular nanotube array organized on the bottom Ti metalfoil. This electrochemical anodization of titanium is a simpleand cost-effective method for growth of highly ordered TiO2

NTs, and uniform titania nanotube arrays of various pore sizes,lengths, and wall thicknesses can be easily grown by tailoringelectrochemical conditions.4 For applications such as solar cells,which require a fast transport of charge carriers, the electro-chemically synthesized regular nanotube array clearly has itsadvantage.

There have been reports11,13 on the nanostructure transforma-tion of TiO2 between nanosheets, nanotubes, nanorods, andnanoparticles during hydrothermal synthesis, based on the pHvariation of chemical solution, but there is no report on thenanostructure transformation for electrochemical anodization-synthesized regular nanotube arrays. In this paper, we reportfor the first time on the transformation of TiO2 NTs grown inethylene glycol + NH4F by electrochemical oxidation intonanoparticles at certain annealing conditions. We found thatTiO2 NTs transform to the nanoparticle phase when annealingis performed at a high-temperature ramping rate with theopening end of NTs sealed by close contact with a supportingplate. This is explained by the catalytic reaction of fluorineresidues in NTs with TiO2. The formed NPs have a truncatedbipyramid shape14 with a high portion of reactive (001) surfacearea.15-17 The size of the NPs depends on the fluorineconcentration and can be controlled within 20-500 nm. Thecrystal quality of the formed anatase NPs is superior to that ofanatase NTs.

Titanium (Ti) sheets with 0.25 mm thickness and 99.97%purity were used for electrochemical oxidation in electrolyteprepared using NH4F (98%) and ethylene glycol (99.8%). Themetal sheets were first cleaned in acetone and ethanol, followedby rinsing in deionized water and drying in a nitrogen stream.Electrochemical anodization was carried out in a DC voltagerange of 30-60 V with the NH4F concentration varied in a rangeof 0.1-1 wt %. A 10% water (H2O) solvent was added to the

* To whom correspondence should be addressed. E-mail: Yahya.Alivov@ttu.edu.

† E-mail: Zhaoyang.Fan@ttu.edu.

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10.1021/jp905174x CCC: $40.75 2009 American Chemical Society

Published on Web 07/01/2009

2009, 113, 12954–12957

electrolyte to increase the growth rate. The use of ethylene glycolwith the addition of water as a solvent in the electrolyte wasfound to dramatically increase the TiO2 nanotube growthrate.18,19 A two-electrode cell was used with a platinum meshedplate as the counter electrode, separated from the titanium anodewith a distance of 2 cm. The obtained highly ordered TiO2

nanotube arrays were annealed in air in the temperature rangeof 300-800 °C. The morphology and geometry (diameter, wallthickness, and height) of the TiO2 nanotubes were studied usingscanning electron microscope (SEM). Figure 1 represents typicalSEM images of the as-grown samples (a) with inset showingthe top views and (b) with the inset showing cross sections atdifferent magnifications. A well-defined tubular structure canbe observed from this figure. The average diameter of the NTsdepended on the anodization voltage, being 80 and 160 nm for30 and 60 V, respectively. The NT wall thickness depended onthe acid concentration and ranged over 10-30 nm. NT arraysgrew on both surfaces of the Ti sheet without noticeabledifferences in the NT film thickness, diameter, wall thickness,and surface morphology. Both sides of the as-grown nanotubeshave a gray color, as shown in Figure 2a. After rapid annealingon a glass plate at 500 °C for 30 min with a ramping rate of 16°C/min, the bottom side facing the glass transformed to yellowcolor, as shown in Figure 2b, while the color of the top sideremained the same (although the amorphous TiO2 was convertedinto the anatase phase). SEM analysis of the yellow color siderevealed a nanoparticle pattern, as shown in Figure 3. In thisfigure (a-c) correspond to top views at different magnifications.Thus, the nanotube array in contact with the glass plate was

transformed into nanoparticles during annealing, while thenanotube array on the other side of the sample without coveringretained its nanotube array morphology. Hereafter, we will referthe sample side with nanoparticles as the NP side, while theside with NT arrays will be referred to as the NT side. The NP

Figure 1. Representative SEM images of TiO2 nanotube arrays grownin ethylene glycol + NH4F electrolyte by electrochemical anodization;(a) and inset, top views at different magnifications; (b) and insert, sideviews at different magnifications.

Figure 2. Pictures of the NT side (top) and the NP sides (bottom) ofthe annealed on glass samples. Size of the images: 0.5 in. × 1 in.

Figure 3. (a-c) SEM images of TiO2 nanoparticle layers at differentmagnifications. Most of the nanoparticles have a shape of a truncatedpyramid and bipyramid, as shown in (c).

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size of the sample shown in the Figure 3 (grown in 0.5% NH4F)was in the range of 300-400 nm. The formed NP layers aremechanically stable and do not change in morphology andthickness even after treatment in ultrasound. This implies thatthe particles have catenated with each other, ensuring, inparticular, good electrical contacts. Most of the nanoparticleshave truncated bipyramid or pyramid shape, as shown in Figure3c. The cross section of the NP side is shown in SupportingInformation Figure 1S. The thickness of the NP side was foundto be 13-17 µm compared to the 56-60 µm thickness of theoriginal NT films. Such a reduction of the NT film thicknessby 4-5 times can be explained by a collapse of the originalhollow NTs with amorphous structure and densification andcrystallization into NPs.

Crystalline and optical properties of the samples were studiedby a glancing angle X-ray diffractometer (GAXRD) andphotoluminescence (PL) methods, which revealed a greatenhancement of these properties for the NP side of the sample.Figure 4 presents the XRD spectra of the NP and NT sides ofthe samples. Both sides have anatase (101), (103), (004), (112),and (200) diffraction peaks at 25.3, 36.95, 37.75, 38.45, and47.95°, respectively. A diffraction peak at 44.55° refers to theTi sheet peak. However, the major anatase peak (101) intensityof the NP side is 2.5 times greater than that of NT side, and thefull width at half-maximum (fwhm) of the NP side of the (101)peak is smaller compared to that of the NT side, being 0.31and 0.38°, respectively. These data indicate that the crystalquality of the NP side is superior to that of the NT side. Theanatase-rutile transformation temperature for the NP side washigher by 100 °C compared to that for the NT side, being 750and 650 °C, respectively.

A series of experiments was performed using differentannealing conditions to understand the mechanism of NT-NPtransformation. It was found that no NPs can be formed whenthe annealing temperature is at or below ∼300 °C, and NTsstarted transforming to NPs at 400 °C with a partial NP patternon NT arrays after a limited annealing time (images are shownin Figure S2 in the Supporting Information). Rapid NT-NPtransition occurs when annealed at 500 °C or above. Thetemperature ramping rate was found to be critical for NT-NPtransition. When the temperature ramping rate was as low as 1°C/min, no NP pattern was observed. Only at ramping rates ashigh as 16 °C/min or above did NT-NP transformation occur.At ramping rates around 10 °C/min, a partial NP phase wasobserved. These results indicate that a high-temperature rampingrate is necessary for NP formation. In further experiments,samples were annealed in a way that both sides were exposedto flowing air without blockage. No NP pattern was observedon any side of the sample after annealing. In contrast, both sidesof the sample had a well-defined NP pattern when samples were

annealed with both sides covered by two glass plates. Theseexperiments showed that “sealing” the opening ends of the NTsis necessary to observe NT-NP transition. Also, there was aminimum NT film thickness to observe the NP transition. Itwas found that when the NT film thickness was ∼10 µm, noNP pattern was observed. When the film thickness was ∼17µm, only a partial NP pattern formed. Only NT arrays withfilm a thickness of 30 µm and above revealed full NT-NPtransformation. The above observations of NT-NP transforma-tion could be explained by the reaction of TiO2 NTs with theelectrolyte residues in the nanotubes after growth. Althoughsamples were rinsed and dried in a nitrogen stream after growth,residual electrolyte could still exist in the long (∼60 µm)nanotubes. The glass plate served as a semibarrier to block therapid evaporation of residual electrolyte during the annealing,and the rapid ramping rate and long nanotube guaranteed thatsome electrolyte remained in the tube at the transformationtemperature (>400 °C).

Considering that the electrolyte is a mixture of NH4F, H2O,and ethylene glycol, further experiments were conducted todetermine the chemical component responsible for the NT-NPtransformation. TiO2 NT samples were first annealed at lowtemperature to completely drive out the electrolyte residuals,and then, they were soaked in ethylene glycol, 0.4% NH4Faqueous solution, or H2O followed by annealing in a semisealedglass container at 500 °C for 30 min. Experiments showed thatonly samples soaked with NH4F aqueous solution transformedto the NP phase. In all other cases (ethylene glycol and H2O),no change of the initial NT pattern was observed. There wasno thickness limitation or temperature ramping rate limitationfor annealing in a sealed container, and all samples wereconverted to NPs. This indicates that the presence of fluorine(F) is necessary for NT-NP transformation.

The NP size was found depending on the amount of fluorinein the NTs during annealing in the sealed glass container. Itdecreased with an increase of F concentration. Specifically, NPsizes from NT samples presoaked in a 0.1, 0.5, 1, and 2% NH4Faqueous solution were, respectively, 220, 110, 35, and 20 nm.(Representative SEM images of NP patterns with different grainsizes are shown in Figure S3 in the Supporting Information.)The NP size was determined by averaging the size of 20nanoparticles. The deviation was estimated to be 20%. In allcases, truncated bipyramid-shaped nanoparticles were observedregardless of nanoparticle size. The similar trend of NP sizedependence on F concentration was observed for samplesannealed on glass plates. The NP size in this case ranged over60-500 nm when the F concentration decreased from 1 to 0.1%.The variation of the F concentration in this case was achievedby using different NH4F concentrations in electrolyte duringelectrochemical growth. The NP size range in this case waslarger than that in the case where annealing was done in a glasscontainer (20-230 nm). Plots showing the dependence of NPsize on F concentration for each case are shown in Figure 5.Thus, by varying the F concentration during heat treatment andusing different annealing methods, the NP size can be controlledwithin 20-500 nm.

The bipyramid nanoparticles with {001} and {101} facetswere observed. A schematic of anatase the TiO2 truncatedbipyramid observed in our work is illustrated in the inset toFigure 5. The degree of pyramid truncation is characterized bya ratio of the B and A parameters (B/A).14,17 Previously, it wasreported that most anatase TiO2 crystals are strongly dominatedby stable {101} facets (by more than 94%). Recently, Yang etal.20 predicted by first-principle calculations and confirmed

Figure 4. GAXRD spectra of the NP side (red) and the NT side (blue)of the TiO2 nanostructure sample.

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experimentally that termination with F atoms yields the lowestvalue of surface energy and makes (001) surfaces more stablecompared to (101) surfaces. This was explained by a balancingof O-O/O-F repulsions and Ti-O/Ti-F attractions, whichstabilizes Ti and O atoms on the surface.20 The B/A parametersof the pyramids in our NPs changed in the range of 0.55-0.82,although not in predictable way. However, the B/A values inour samples are much higher than the predicted maximum valueof 0.3714 when no surface energy modification atoms are present.It is known that the (001) surface is more photocatalyticallyactive than the (101) surface.15,21,22 However, what forces drivethe NTs in ambient fluorine to contract and further to breakdown to form nanoparticles? It is well-known that the equilib-rium shape of a crystal is dictated by the minimization of surfaceenergy. It is likely that F termination of the TiO2 NT surface isnot energetically favorable at high temperatures, which resultsin NT contraction, then in breaking, and then in formingnanoparticles. However, whether this is a true mechanism ornot requires further investigations.

In conclusion, nanotube-nanoparticle transition was observedafter annealing of NT films in fluorine ambient. The NP sizecan be controlled in the range 20-500 nm by changing fluorineconcentration at annealing. The formed NPs have enhancedcrystal properties compared to NT arrays and have a truncated

pyramid shape with large ratio of photocatalytically active (001)surface area.

Acknowledgment. This work was supported by Texas TechUniversity and the U. S. Army CERDEC (Contract No.W15P7T-07-D-P040).

Supporting Information Available: Cross-sectional viewof the NP layer, top views of a partial NP sample at differentmagnifications, and representative SEM images of different sizeNPs formed from NTs using different NH4F concentrations. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

References and Notes

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1991, 45, 921.(13) Nian, J. N.; Teng, H. J. Phys. Chem. B 2006, 110, 4193.(14) Barnard, A. S.; Curtiss, L. A. Nano Lett. 2005, 5, 1261.(15) Gong, X. Q.; Selloni, A. J. Phys. Chem. B 2005, 109, 19560.(16) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gratzel, M. Phys. ReV.

Lett. 1998, 81, 2954.(17) Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. ReV. B 2001, 63,

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O. K.; Mor, G. K. J. Phys. Chem. B 2006, 110, 16179.(19) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose,

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JP905174X

Figure 5. Dependence of NP size on fluorine concentration duringannealing of NT samples in a sealed glass container and on glass plate.Inset: A sketch of truncated bipyramid-shaped TiO2 NPs showing (001)and (101) facets; A and B are length parameters, as illustrated in thefigure.

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